README.md

# Daily Coding Problem
### [Problem 1](solutions/problem_001.js)
#### Difficulty: `Easy`

This problem was recently asked by Google.

Given a list of numbers and a number `k`, return whether any two numbers from the list add up to `k`.

For example, given `[10, 15, 3, 7]` and `k` of `17`, return true since `10 + 7` is `17`.

Bonus: Can you do this in one pass?

---
### [Problem 2](solutions/problem_002.js)
#### Difficulty: `Hard`

This problem was asked by Uber.

Given an array of integers, return a new array such that each element at index `i` of the new array is the product of all the numbers in the original array except the one at `i`.

For example, if our input was `[1, 2, 3, 4, 5]`, the expected output would be `[120, 60, 40, 30, 24]`. If our input was `[3, 2, 1]`, the expected output would be `[2, 3, 6]`.

Follow-up: what if you can't use division?

---
### [Problem 3](solutions/problem_003.js)
#### Difficulty: `Medium`

This problem was asked by Google.

Given the root to a binary tree, implement `serialize(root)`, which serializes the tree into a string, and `deserialize(s)`, which deserializes the string back into the tree.

For example, given the following `Node` class:

```javascript
class Node {
  constructor(val, left=null, right=null) {
    this.val = val;
    this.left = left;
    this.right = right;
  }
}
```

The following test should pass:
```javascript
const node = new Node('root', new Node('left', new Node('left.left')), new  Node('right'));
console.log(deserialize(serialize(node)).left.left.val === 'left.left');
```

---
### [Problem 4](solutions/problem_004.js)
#### Difficulty: `Hard`

This problem was asked by Stripe.

Given an array of integers, find the first missing positive integer in linear time and constant space. In other words, find the lowest positive integer that does not exist in the array. The array can contain duplicates and negative numbers as well.

For example, the input `[3, 4, -1, 1]` should give `2`. The input `[1, 2, 0]`should give `3`.

You can modify the input array in-place.

---
### [Problem 5](solutions/problem_005.js)
#### Difficulty: `Medium`

This problem was asked by Jane Street.

`cons(a, b)` constructs a pair, and `car(pair)` and `cdr(pair)` returns the first and last element of that pair. For example, `car(cons(3, 4))` returns `3`, and `cdr(cons(3, 4))` returns `4`.

Given this implementation of cons:

```javascript
const cons = (a, b) => {
  const pair = (f) => {
    return f(a,b);
  }
  return pair;
}
```
---
### [Problem 6](solutions/problem_006.js)
#### Difficulty: `Hard`

This problem was asked by Google.

An XOR linked list is a more memory efficient doubly linked list. Instead of each node holding `next` and `prev` fields, it holds a field named `both`, which is an XOR of the next node and the previous node. Implement an XOR linked list; it has an `add(element)` which adds the element to the end, and a `get(index)` which returns the node at index.

If using a language that has no pointers (such as Python), you can assume you have access to `get_pointer` and `dereference_pointer` functions that converts between nodes and memory addresses.

---
### [Problem 7](solutions/problem_007.js)
#### Difficulty: `Medium`

This problem was asked by Facebook.

Given the mapping `a = 1, b = 2, ... z = 26`, and an encoded message, count the number of ways it can be decoded.

For example, the message `111` would give `3`, since it could be decoded as `aaa`, `ka`, and `ak`.

You can assume that the messages are decodable. For example, `001` is not allowed.

---
### [Problem 8](solutions/problem_008.js)
#### Difficulty: `Easy`

This problem was asked by Google.

A unival tree (which stands for "universal value") is a tree where all nodes under it have the same value.

Given the root to a binary tree, count the number of unival subtrees.

For example, the following tree has `5` unival subtrees:

```
  0
 / \
1   0
   / \
  1   0
 / \
1   1
```

---
### [Problem 9](solutions/problem_009.js)
#### Difficulty: `Hard`

This problem was asked by Airbnb.

Given a list of integers, write a function that returns the largest sum of non-adjacent numbers. Numbers can be `0` or `negative`.

For example, `[2, 4, 6, 8]` should return `12`, since we pick `4` and `8`. `[5, 1, 1, 5]` should return `10`, since we pick `5` and `5`.

Follow-up: Can you do this in `O(N)` time and constant space?

---
### [Problem 10](solutions/problem_010.js)
#### Difficulty: `Medium`

This problem was asked by Apple.

  Implement a job scheduler which takes in a function `f` and an integer `n`, and calls `f` after `n` milliseconds.

---
### [Problem 11](solutions/problem_011.js)
#### Difficulty: `Medium`

This problem was asked by Twitter.

Implement an autocomplete system. That is, given a query string `s` and a set of all possible query strings, return all strings in the set that have `s` as a prefix.

For example, given the query string `de` and the set of strings [`dog`, `deer`, `deal`], return [`deer`, `deal`].

Hint: Try preprocessing the dictionary into a more efficient data structure to speed up queries.

---
### [Problem 12](solutions/problem_012.js)
#### Difficulty: `Hard`

This problem was asked by Amazon.

There exists a staircase with `N` steps, and you can climb up either `1` or `2` steps at a time. Given `N`, write a function that returns the number of unique ways you can climb the staircase. The order of the steps matters.

For example, if `N` is `4`, then there are `5` unique ways:
- `1, 1, 1, 1`
- `2, 1, 1`
- `1, 2, 1`
- `1, 1, 2`
- `2, 2`

What if, instead of being able to climb 1 or 2 steps at a time, you could climb any number from a set of positive integers X? For example, if X = `{1, 3, 5}`, you could climb `1`, `3`, or `5` steps at a time.

---
### [Problem 13](solutions/problem_013.js)
#### Difficulty: `Hard`

This problem was asked by Amazon.

Given an integer `k` and a string `s`, find the length of the longest substring that contains at most `k` distinct characters.

For example, given `s` = `abcba` and `k` = `2`, the longest substring with k distinct characters is `bcb`.

---
### [Problem 14](solutions/problem_014.js)
#### Difficulty: `Medium`

This problem was asked by Google.

The area of a circle is defined as `πr²`. Estimate `π` to `3` decimal places using a `Monte Carlo` method. Hint: The basic equation of a circle is `x² + y² = r²`.

---
### [Problem 15](solutions/problem_015.js)
#### Difficulty: `Medium`

This problem was asked by Facebook.

Given a stream of elements too large to store in memory, pick a random element from the stream with uniform probability.

---
### [Problem 16](solutions/problem_016.js)
#### Difficulty: `Easy`

This problem was asked by Twitter.

You run an e-commerce website and want to record the last `N` order ids in a log. Implement a data structure to accomplish this, with the following API:

- `record(order_id)`: adds the order_id to the log
- `get_last(i)`: gets the ith last element from the log. `i` is guaranteed to be smaller than or equal to `N`. You should be as efficient with time and space as possible.

You should be as efficient with time and space as possible.

---
### [Problem 17](solutions/problem_017.js)
#### Difficulty: `Hard`

This problem was asked by Google.

Suppose we represent our file system by a string in the following manner:

The string `"dir\n\tsubdir1\n\tsubdir2\n\t\tfile.ext"` represents:

```
dir
    subdir1
    subdir2
        file.ext
```

The directory `dir` contains an empty sub-directory `subdir1` and a sub-directory `subdir2` containing a file `file.ext`.

The string `"dir\n\tsubdir1\n\t\tfile1.ext\n\t\tsubsubdir1\n\tsubdir2\n\t\tsubsubdir2\n\t\t\tfile2.ext"` represents:

```
dir
    subdir1
        file1.ext
        subsubdir1
    subdir2
        subsubdir2
            file2.ext
```

The directory `dir` contains two sub-directories `subdir1` and `subdir2`. `subdir1` contains a file `file1.ext` and an empty second-level sub-directory `subsubdir1`. `subdir2` contains a second-level sub-directory `subsubdir2` containing a file `file2.ext`.

We are interested in finding the longest (number of characters) absolute path to a file within our file system. For example, in the second example above, the longest absolute path is `"dir/subdir2/subsubdir2/file2.ext"`, and its length is 32 (not including the double quotes).

Given a string representing the file system in the above format, return the length of the longest absolute path to a file in the abstracted file system. If there is no file in the system, return `0`.

Note:

The name of a file contains at least a period and an extension.

The name of a directory or sub-directory will not contain a period.

---
### [Problem 18](solutions/problem_018.js)
#### Difficulty: `Hard`

This problem was asked by Google.

Given an array of integers and a number `k`, where 1 <= k <= length of the array, compute the maximum values of each subarray of length k.

For example, given array = `[10, 5, 2, 7, 8, 7]` and `k = 3`, we should get: `[10, 7, 8, 8]`, since:

- 10 = `max(10, 5, 2)`
- 7 = `max(5, 2, 7)`
- 8 = `max(2, 7, 8)`
- 8 = `max(7, 8, 7)`

Do this in `O(n)` time and `O(k)` space. You can modify the input array in-place and you do not need to store the results. You can simply print them out as you compute them.

---
### [Problem 19](solutions/problem_019.js)
#### Difficulty: `Medium`

This problem was asked by Facebook.

A builder is looking to build a row of `N` houses that can be of `K` different colors. He has a goal of minimizing cost while ensuring that no two neighboring houses are of the same color.

Given an `N` by `K` matrix where the `nth` row and `kth` column represents the cost to build the `nth` house with `kth` color, return the minimum cost which achieves this goal.

---
### [Problem 20](solutions/problem_020.js)
#### Difficulty: `Easy`

This problem was asked by Google.

Given two singly linked lists that intersect at some point, find the intersecting node. The lists are non-cyclical.

For example, given A = 3 -> 7 -> 8 -> 10 and B = 99 -> 1 -> 8 -> 10, return the node with value 8.

In this example, assume nodes with the same value are the exact same node objects.

Do this in O(M + N) time (where M and N are the lengths of the lists) and constant space.

---
### [Problem 21](solutions/problem_021.js)
#### Difficulty: `Easy`

This problem was asked by Snapchat.

Given an array of time intervals (start, end) for classroom lectures (possibly overlapping), find the minimum number of rooms required.

For example, given [`(30, 75)`, `(0, 50)`, `(60, 150)`], you should return `2`.

---
### [Problem 22](solutions/problem_022.js)
#### Difficulty: `Medium`

This problem was asked by Microsoft.

Given a dictionary of words and a string made up of those words (no spaces), return the original sentence in a list. If there is more than one possible reconstruction, return any of them. If there is no possible reconstruction, then return null.

For example, given the set of words '`quick`', '`brown`', '`the`', '`fox`', and the string "`thequickbrownfox`", you should return ['the', 'quick', 'brown', 'fox'].

Given the set of words 'bed', 'bath', 'bedbath', 'and', 'beyond', and the string "bedbathandbeyond", return either `['bed', 'bath', 'and', 'beyond]` or `['bedbath', 'and', 'beyond']`.

---
### [Problem 23](solutions/problem_023.js)
#### Difficulty: `Easy`

This problem was asked by Google.

You are given an M by N matrix consisting of booleans that represents a board. Each True boolean represents a wall. Each False boolean represents a tile you can walk on.

Given this matrix, a start coordinate, and an end coordinate, return the minimum number of steps required to reach the end coordinate from the start. If there is no possible path, then return null. You can move up, left, down, and right. You cannot move through walls. You cannot wrap around the edges of the board.

For example, given the following board:

```
[[f, f, f, f],
[t, t, f, t],
[f, f, f, f],
[f, f, f, f]]
```
and start = `(3, 0)` (bottom left) and end = `(0, 0)` (top left), the minimum number of steps required to reach the end is 7, since we would need to go through `(1, 2)` because there is a wall everywhere else on the second row.

---
### [Problem 24](solutions/problem_024.js)
#### Difficulty: `Medium`

This problem was asked by Google.

Implement locking in a binary tree. A binary tree node can be locked or unlocked only if all of its descendants or ancestors are not locked.

Design a binary tree node class with the following methods:
- `is_locked`, which returns whether the node is locked
- `lock`, which attempts to lock the node. If it cannot be locked, then it should return false. Otherwise, it should lock it and return true.
- `unlock`, which unlocks the node. If it cannot be unlocked, then it should return false. Otherwise, it should unlock it and return true.

You may augment the node to add parent pointers or any other property you would like. You may assume the class is used in a single-threaded program, so there is no need for actual locks or mutexes. Each method should run in `O(h)`, where h is the height of the tree.

---
### [Problem 25](solutions/problem_025.js)
#### Difficulty: `Hard`

This problem was asked by Facebook.

Implement regular expression matching with the following special characters:

- `.` (period) which matches any single character
- `*` (asterisk) which matches zero or more of the preceding element

That is, implement a function that takes in a string and a valid regular expression and returns whether or not the string matches the regular expression.

For example, given the regular expression "`ra.`" and the string "`ray`", your function should return `true`. The same regular expression on the string "`raymond`" should return `false`.

Given the regular expression "`.*at`" and the string "`chat`", your function should return `true`. The same regular expression on the string "`chats`" should return `false`.

---
### [Problem 26](solutions/problem_026.js)
#### Difficulty: `Medium`

This problem was asked by Google.

Given a singly linked list and an integer `k`, remove the `k`th last element from the list. `k` is guaranteed to be smaller than the length of the list.

The list is very long, so making more than one pass is prohibitively expensive.

Do this in constant space and in one pass.

---
### [Problem 27](solutions/problem_027.js)
#### Difficulty: `Easy`

This problem was asked by Facebook.

Given a string of round, curly, and square open and closing brackets, return whether the brackets are balanced (well-formed).

For example, given the string `([])[]({})`, you should return `true`.

Given the string `([)]` or `((()`, you should return `false`.

---
### [Problem 28](solutions/problem_028.js)
#### Difficulty: `Medium`

This problem was asked by Palantir.

Write an algorithm to justify text. Given a sequence of words and an integer line length k, return a list of strings which represents each line, fully justified.

More specifically, you should have as many words as possible in each line. There should be at least one space between each word. Pad extra spaces when necessary so that each line has exactly length k. Spaces should be distributed as equally as possible, with the extra spaces, if any, distributed starting from the left.

If you can only fit one word on a line, then you should pad the right-hand side with spaces.

Each word is guaranteed not to be longer than k.

For example, given the list of words `["the", "quick", "brown", "fox", "jumps", "over", "the", "lazy", "dog"]` and `k` = 16, you should return the following:

```
["the  quick brown", # 1 extra space on the left
"fox  jumps  over", # 2 extra spaces distributed evenly
"the   lazy   dog"] # 4 extra spaces distributed evenly
```

---
### [Problem 29](solutions/problem_029.js)
#### Difficulty: `Easy`

This problem was asked by Amazon.

Run-length encoding is a fast and simple method of encoding strings. The basic idea is to represent repeated successive characters as a single count and character. For example, the string `AAAABBBCCDAA` would be encoded as `4A3B2C1D2A`.

Implement run-length encoding and decoding. You can assume the string to be encoded have no digits and consists solely of alphabetic characters. You can assume the string to be decoded is valid.

---
### [Problem 30](solutions/problem_030.js)
#### Difficulty: `Medium`

This problem was asked by Facebook.

You are given an array of non-negative integers that represents a two-dimensional elevation map where each element is unit-width wall and the integer is the height. Suppose it will rain and all spots between two walls get filled up.

Compute how many units of water remain trapped on the map in `O(N)` time and `O(1)` space.

For example, given the input `[2, 1, 2]`, we can hold `1` unit of water in the middle.

Given the input `[3, 0, 1, 3, 0, 5]`, we can hold `3` units in the first index, `2` in the second, and `3` in the fourth index (we cannot hold 5 since it would run off to the left), so we can trap `8` units of water.

---
### [Problem 31](solutions/problem_031.js)
#### Difficulty: `Easy`

This problem was asked by Google.

The edit distance between two strings refers to the minimum number of character insertions, deletions, and substitutions required to change one string to the other. For example, the edit distance between `kitten` and `sitting` is three: substitute the `k` for `s`, substitute the `e` for `i`, and append a `g`.

Given two strings, compute the edit distance between them.

---
### [Problem 32](solutions/problem_032.js)
#### Difficulty: `Hard`

This problem was asked by Jane Street.

Suppose you are given a table of currency exchange rates, represented as a 2D array. Determine whether there is a possible arbitrage: that is, whether there is some sequence of trades you can make, starting with some amount A of any currency, so that you can end up with some amount greater than A of that currency.

There are no transaction costs and you can trade fractional quantities.

---
### [Problem 33](solutions/problem_033.js)
#### Difficulty: `Easy`

This problem was asked by Microsoft.

Compute the running median of a sequence of numbers. That is, given a stream of numbers, print out the median of the list so far on each new element.

Recall that the median of an even-numbered list is the average of the two middle numbers.

For example, given the sequence `[2, 1, 5, 7, 2, 0, 5]`, your algorithm should print out:

```text
2
1.5
2
3.5
2
2
2
```

---
### [Problem 34](solutions/problem_034.js)
#### Difficulty: `Medium`

This problem was asked by Quora.

Given a string, find the palindrome that can be made by inserting the fewest number of characters as possible anywhere in the word. If there is more than one palindrome of minimum length that can be made, return the lexicographically earliest one (the first one alphabetically).

For example, given the string `race`, you should return `ecarace`, since we can add three letters to it (which is the smallest amount to make a palindrome). There are seven other palindromes that can be made from `race` by adding three letters, but `ecarace` comes first alphabetically.

As another example, given the string `google`, you should return `elgoogle`.

---
### [Problem 35](solutions/problem_035.js)
#### Difficulty: `Hard`

This problem was asked by Google.

Given an array of strictly the characters 'R', 'G', and 'B', segregate the values of the array so that all the Rs come first, the Gs come second, and the Bs come last. You can only swap elements of the array.

Do this in linear time and in-place.

For example, given the array `['G', 'B', 'R', 'R', 'B', 'R', 'G']`, it should become `['R', 'R', 'R', 'G', 'G', 'B', 'B']`.

---
### [Problem 36](solutions/problem_036.js)
#### Difficulty: `Medium`

This problem was asked by Dropbox.

Given the root to a binary search tree, find the second largest node in the tree.

---
### [Problem 37](solutions/problem_037.js)
#### Difficulty: `Easy`

This problem was asked by Google.

The power set of a set is the set of all its subsets. Write a function that, given a set, generates its power set.

For example, given the set `{1, 2, 3}`, it should return `{{}, {1}, {2}, {3}, {1, 2}, {1, 3}, {2, 3}, {1, 2, 3}}`.

You may also use a list or array to represent a set.

---
### [Problem 38](solutions/problem_038.js)
#### Difficulty: `Hard`

This problem was asked by Microsoft.

You have an N by N board. Write a function that, given N, returns the number of possible arrangements of the board where N queens can be placed on the board without threatening each other, i.e. no two queens share the same row, column, or diagonal.

---
### [Problem 39](solutions/problem_039.js)
#### Difficulty: `Medium`

This problem was asked by Dropbox.

Conway's Game of Life takes place on an infinite two-dimensional board of square cells. Each cell is either dead or alive, and at each tick, the following rules apply:

- Any live cell with less than two live neighbours dies.
- Any live cell with two or three live neighbours remains living.
- Any live cell with more than three live neighbours dies.
- Any dead cell with exactly three live neighbours becomes a live cell.

A cell neighbours another cell if it is horizontally, vertically, or diagonally adjacent.

Implement Conway's Game of Life. It should be able to be initialized with a starting list of live cell coordinates and the number of steps it should run for. Once initialized, it should print out the board state at each step. Since it's an infinite board, print out only the relevant coordinates, i.e. from the top-leftmost live cell to bottom-rightmost live cell.

You can represent a live cell with an asterisk (`*`) and a dead cell with a dot (`.`).

---
### [Problem 40](solutions/problem_040.js)
#### Difficulty: `Hard`

This problem was asked by Google.

Given an array of integers where every integer occurs three times except for one integer, which only occurs once, find and return the non-duplicated integer.

For example, given `[6, 1, 3, 3, 3, 6, 6]`, return `1`. Given `[13, 19, 13, 13]`, return `19`.

Do this in `O(N)` time and `O(1)` space.

---
### [Problem 41](solutions/problem_041.js)
#### Difficulty: `Medium`

This problem was asked by Facebook.

Given an unordered list of flights taken by someone, each represented as (origin, destination) pairs, and a starting airport, compute the person's itinerary. If no such itinerary exists, return `null`. If there are multiple possible itineraries, return the lexicographically smallest one. All flights must be used in the itinerary.

For example, given the list of flights `[('SFO', 'HKO'), ('YYZ', 'SFO'), ('YUL', 'YYZ'), ('HKO', 'ORD')]` and starting airport `YUL`, you should return the list `['YUL', 'YYZ', 'SFO', 'HKO', 'ORD']`.

Given the list of flights `[('SFO', 'COM'), ('COM', 'YYZ')]` and starting airport `COM`, you should return `null`.

Given the list of flights `[('A', 'B'), ('A', 'C'), ('B', 'C'), ('C', 'A')]` and starting airport `A`, you should return the list `['A', 'B', 'C', 'A', 'C']` even though `['A', 'C', 'A', 'B', 'C']` is also a valid itinerary. However, the first one is lexicographically smaller.

---
### [Problem 42](solutions/problem_042.js)
#### Difficulty: `Hard`

This problem was asked by Google.

Given a list of integers `S` and a target number `k`, write a function that returns a subset of `S` that adds up to `k`. If such a subset cannot be made, then return `null`.

Integers can appear more than once in the list. You may assume all numbers in the list are positive.

For example, given S = `[12, 1, 61, 5, 9, 2]` and k = `24`, return `[12, 9, 2, 1]` since it sums up to `24`.

---
### [Problem 43](solutions/problem_043.js)
#### Difficulty: `Easy`

This problem was asked by Amazon.

Implement a stack that has the following methods:

- `push(val)`, which pushes an element onto the stack
- `pop()`, which pops off and returns the topmost element of the stack. If there are no elements in the stack, then it should throw an error or return null.
- `max()`, which returns the maximum value in the stack currently. If there are no elements in the stack, then it should throw an error or return null.

Each method should run in constant time.

---
### [Problem 44](solutions/problem_044.js)
#### Difficulty: `Medium`

This problem was asked by Google.

We can determine how "out of order" and array `A` is by counting the number of inversions it has. Two elements `A[i]` and `A[j]` form an inversion if `A[i] > A[j]` but `i < j`. That is, a smaller elements appears after the larger element

Given an array, account the number of inversions it has. Do this faster than `O(N^2)` time.

You may assume each element in the array is distinct.

For example a sorted list has zero inversions. The array `[2, 4, 1, 3, 5]` has three inversions `(2,1)`, `(4,1)` and `(4,3)`. The array `[5, 4, 3, 2, 1]` has ten inversions: every distinct pair forms an inversion.

---
### [Problem 45](solutions/problem_045.js)
#### Difficulty: `Easy`

This problem was asked by Two Sigma.

Using a function `rand5()` that returns an integer from `1` to `5` (inclusive) with uniform probability, implement a function `rand7()` that returns an integer from `1` to `7` (inclusive).

---
### [Problem 46](solutions/problem_046.js)
#### Difficulty: `Medium`

This problem was asked by Amazon.

Given a string, find the longest palindromic contiguous substring. If there are more than one with the maximum length, return any one.

For example, the longest palindromic substring of `aabcdcb` is `bcdcb`. The longest palindromic substring of `bananas` is `anana`.

---
### [Problem 47](solutions/problem_047.js)
#### Difficulty: `Easy`

This problem was asked by Facebook.

Given a array of numbers representing the stock prices of a company in chronological order, write a function that calculates the maximum profit you could have made from buying and selling that stock once. You must buy before you can sell it.

For example, given `[9, 11, 8, 5, 7, 10]`, you should return `5`, since you could buy the stock at `5` dollars and sell it at `10` dollars.

---
### [Problem 48](solutions/problem_048.js)
#### Difficulty: `Medium`

This problem was asked by Google.

Given pre-order and in-order traversals of a binary tree, write a function to reconstruct the tree.

For example, given the following preorder traversal:

`[a, b, d, e, c, f, g]`

And the following inorder traversal:

`[d, b, e, a, f, c, g]`

You should return the following tree:

```
     a
    / \
   b   c
  / \ / \
 d  e f  g
```

---
### [Problem 49](solutions/problem_049.js)
#### Difficulty: `Medium`

This problem was asked by Amazon.

Given an array of numbers, find the maximum sum of any contiguous subarray of the array.

For example, given the array `[34, -50, 42, 14, -5, 86]`, the maximum sum would be `137`, since we would take elements `42`, `14`, `-5`, and `86`.

Given the array `[-5, -1, -8, -9]`, the maximum sum would be `0`, since we would not take any elements.

Do this in `O(N)` time.

---
### [Problem 50](solutions/problem_050.js)
#### Difficulty: `Easy`

This problem was asked by Microsoft.

Suppose an arithmetic expression is given as a binary tree. Each leaf is an integer and each internal node is one of '`+`', '`−`', '`∗`', or '`/`'.

Given the root to such a tree, write a function to evaluate it.

For example, given the following tree:

```
     *
    / \
   +    +
  / \  / \
 3  2  4  5
```

You should return `45`, as it is `(3 + 2) * (4 + 5)`.

---
### [Problem 51](solutions/problem_051.js)
#### Difficulty: `Medium`

This problem was asked by Facebook.

Given a function that generates perfectly random numbers between `1` and `k` (inclusive), where `k` is an input, write a function that shuffles a deck of cards represented as an array using only swaps.

It should run in `O(N)` time.

Hint: Make sure each one of the `52!` permutations of the deck is equally likely.

---
### [Problem 52](solutions/problem_052.js)
#### Difficulty: `Hard`

This problem was asked by Google.

Implement an LRU (Least Recently Used) cache. It should be able to be initialized with a cache size `n`, and contain the following methods:

- `set(key, value)`: sets key to value. If there are already `n` items in the cache and we are adding a new item, then it should also remove the least recently used item.
- `get(key)`: gets the value at key. If no such key exists, return `null`.

Each operation should run in `O(1)` time.

---
### [Problem 53](solutions/problem_053.js)
#### Difficulty: `Medium`

This problem was asked by Apple.

Implement a queue using two stacks. Recall that a queue is a FIFO (first-in, first-out) data structure with the following methods: `enqueue`, which inserts an element into the queue, and `dequeue`, which removes it.

---
### [Problem 54](solutions/problem_054.js)
#### Difficulty: `Hard`

This problem was asked by Dropbox.

Sudoku is a puzzle where you're given a partially-filled 9 by 9 grid with digits. The objective is to fill the grid with the constraint that every row, column, and box (3 by 3 subgrid) must contain all of the digits from 1 to 9.

Implement an efficient sudoku solver.

---
### [Problem 55](solutions/problem_055.js)
#### Difficulty: `Easy`

This problem was asked by Microsoft.

Implement a URL shortener with the following methods:

- `shorten(url)`, which shortens the url into a six-character alphanumeric string, such as `zLg6wl`.
- `restore(short)`, which expands the shortened string into the original url. If no such shortened string exists, return `null`.

Hint: What if we enter the same URL twice?

---
### [Problem 56](solutions/problem_056.js)
#### Difficulty: `Medium`

This problem was asked by Google.

Given an undirected graph represented as an adjacency matrix and an integer `k`, write a function to determine whether each vertex in the graph can be colored such that no two adjacent vertices share the same color using at most `k` colors.

---
### [Problem 57](solutions/problem_057.js)
#### Difficulty: `Medium`

This problem was asked by Amazon.

Given a string `s` and an integer `k`, break up the string into multiple lines such that each line has a length of `k` or less. You must break it up so that words don't break across lines. Each line has to have the maximum possible amount of words. If there's no way to break the text up, then return `null`.

You can assume that there are no spaces at the ends of the string and that there is exactly one space between each word.

For example, given the string `"the quick brown fox jumps over the lazy dog"` and `k` = 10, you should return: `["the quick", "brown fox", "jumps over", "the lazy", "dog"]`. No string in the list has a length of more than `10`.

---
### [Problem 58](solutions/problem_058.js)
#### Difficulty: `Medium`

This problem was asked by Amazon.

An sorted array of integers was rotated an unknown number of times.

Given such an array, find the index of the element in the array in faster than linear time. If the element doesn't exist in the array, return `null`.

For example, given the array `[13, 18, 25, 2, 8, 10]` and the element `8`, return `4` (the index of `8` in the array).

You can assume all the integers in the array are unique.

---
### [Problem 59](solutions/problem_059.js)
#### Difficulty: `Hard`

This problem was asked by Google.

Implement a file syncing algorithm for two computers over a low-bandwidth network. What if we know the files in the two computers are mostly the same?

---
### [Problem 60](solutions/problem_060.js)
#### Difficulty: `Medium`

This problem was asked by Facebook.

Given a multiset of integers, return whether it can be partitioned into two subsets whose sums are the same.

For example, given the multiset `{15, 5, 20, 10, 35, 15, 10}`, it would return `true`, since we can split it up into `{15, 5, 10, 15, 10}` and `{20, 35}`, which both add up to `55`.

Given the multiset `{15, 5, 20, 10, 35}`, it would return `false`, since we can't split it up into two subsets that add up to the same sum.

---
### [Problem 61](solutions/problem_061.js)
#### Difficulty: `Medium`

This problem was asked by Google.

Implement integer exponentiation. That is, implement the `pow(x, y)` function, where `x` and `y` are integers and returns `x^y`.

Do this faster than the naive method of repeated multiplication.

For example, `pow(2, 10)` should return `1024`.

---
### [Problem 62](solutions/problem_062.js)
#### Difficulty: `Medium`

This problem was asked by Facebook.

There is an `N` by `M` matrix of zeroes. Given `N` and `M`, write a function to count the number of ways of starting at the top-left corner and getting to the bottom-right corner. You can only move right or down.

For example, given a `2` by `2` matrix, you should return `2`, since there are two ways to get to the bottom-right:

- Right, then down
- Down, then right

Given a 5 by 5 matrix, there are `70` ways to get to the bottom-right.

---
### [Problem 63](solutions/problem_063.js)
#### Difficulty: `Easy`

This problem was asked by Microsoft.

Given a 2D matrix of characters and a target word, write a function that returns whether the word can be found in the matrix by going left-to-right, or up-to-down.

For example, given the following matrix:

```
[['F', 'A', 'C', 'I'],
 ['O', 'B', 'Q', 'P'],
 ['A', 'N', 'O', 'B'],
 ['M', 'A', 'S', 'S']]
```

and the target word 'FOAM', you should return true, since it's the leftmost column. Similarly, given the target word 'MASS', you should return true, since it's the last row.

---
### [Problem 64](solutions/problem_064.js)
#### Difficulty: `Hard`

This problem was asked by Google.

A knight's tour is a sequence of moves by a knight on a chessboard such that all squares are visited once.

Given `N`, write a function to return the number of knight's tours on an `N` by `N` chessboard.

---
### [Problem 65](solutions/problem_065.js)
#### Difficulty: `Easy`

This problem was asked by Amazon.

Given a N by M matrix of numbers, print out the matrix in a clockwise spiral.

For example, given the following matrix:

```
[
 [1,  2,  3,  4,  5],
 [6,  7,  8,  9,  10],
 [11, 12, 13, 14, 15],
 [16, 17, 18, 19, 20]
]
```

You should print out the following:

```
1
2
3
4
5
10
15
20
19
18
17
16
11
6
7
8
9
14
13
12
```

---
### [Problem 66](solutions/problem_066.js)
#### Difficulty: `Medium`

This problem was asked by Square.

Assume you have access to a function `toss_biased()` which returns `0` or `1` with a probability that's not 50-50 (but also not 0-100 or 100-0). You do not know the bias of the coin.

Write a function to simulate an unbiased coin toss.

---
### [Problem 67](solutions/problem_067.js)
#### Difficulty: `Hard`

This problem was asked by Google.

Implement an LFU (Least Frequently Used) cache. It should be able to be initialized with a cache size n, and contain the following methods:

- `set(key, value)`: sets key to value. If there are already n items in the cache and we are adding a new item, then it should also remove the least frequently used item. If there is a tie, then the least recently used key should be removed.
- `get(key)`: gets the value at key. If no such key exists, return `null`.

Each operation should run in `O(1)` time.

---
### [Problem 68](solutions/problem_068.js)
#### Difficulty: `Medium`

This problem was asked by Google.

On our special chessboard, two bishops attack each other if they share the same diagonal. This includes bishops that have another bishop located between them, i.e. bishops can attack through pieces.

You are given `N` bishops, represented as (row, column) tuples on a `M` by `M` chessboard. Write a function to count the number of pairs of bishops that attack each other. The ordering of the pair doesn't matter: `(1, 2)` is considered the same as `(2, 1)`.

For example, given `M` = 5 and the list of bishops:

- (0, 0)
- (1, 2)
- (2, 2)
- (4, 0)

The board would look like this:
```
[b 0 0 0 0]
[0 0 b 0 0]
[0 0 b 0 0]
[0 0 0 0 0]
[b 0 0 0 0]
```

You should return `2`, since bishops `1` and `3` attack each other, as well as bishops `3` and `4`.

---
### [Problem 69](solutions/problem_069.js)
#### Difficulty: `Easy`

This problem was asked by Facebook.

Given a list of integers, return the largest product that can be made by multiplying any three integers.

For example, if the list is `[-10, -10, 5, 2]`, we should return `500`, since that's `-10 * -10 * 5`.

You can assume the list has at least three integers.

---
### [Problem 70](solutions/problem_070.js)
#### Difficulty: `Easy`

This problem was asked by Microsoft.

A number is considered perfect if its digits sum up to exactly `10`.

Given a positive integer `n`, return the n-th perfect number.

For example, given `1`, you should return `19`. Given `2`, you should return `28`.

---
### [Problem 71](solutions/problem_071.js)
#### Difficulty: `Easy`

This problem was asked by Two Sigma.

Using a function `rand7()` that returns an integer from `1` to `7` (inclusive) with uniform probability, implement a function `rand5()` that returns an integer from `1` to `5` (inclusive).

---
### [Problem 72](solutions/problem_072.js)
#### Difficulty: `Hard`

This problem was asked by Google.

In a directed graph, each node is assigned an uppercase letter. We define a path's value as the number of most frequently-occurring letter along that path. For example, if a path in the graph goes through "ABACA", the value of the path is 3, since there are 3 occurrences of 'A' on the path.

Given a graph with `n` nodes and `m` directed edges, return the largest value path of the graph. If the largest value is infinite, then return `null`.

The graph is represented with a string and an edge list. The `i`-th character represents the uppercase letter of the `i`-th node. Each tuple in the edge list `(i, j)` means there is a directed edge from the `i`-th node to the `j`-th node. Self-edges are possible, as well as multi-edges.

For example, the following input graph:

`ABACA`

```
[(0, 1),
 (0, 2),
 (2, 3),
 (3, 4)]
```

Would have maximum value 3 using the path of vertices

`[0, 2, 3, 4]`, `(A, A, C, A)`.

The following input graph:
`A`

`[(0, 0)]`

Should return null, since we have an infinite loop.

---
### [Problem 73](solutions/problem_073.js)
#### Difficulty: `Easy`

This problem was asked by Google.

Given the head of a singly linked list, reverse it in-place.

---
### [Problem 74](solutions/problem_074.js)
#### Difficulty: `Medium`

This problem was asked by Apple.

Suppose you have a multiplication table that is `N` by `N`. That is, a 2D array where the value at the `i`-th row and `j`-th column is `(i + 1) * (j + 1)` (if `0`-indexed) or `i * j` (if 1-indexed).

Given integers `N` and `X`, write a function that returns the number of times `X` appears as a value in an `N` by `N` multiplication table.

For example, given `N` = 6 and `X` = 12, you should return `4`, since the multiplication table looks like this:

| 1 | 2 | 3 | 4 | 5 | 6 |

| 2 | 4 | 6 | 8 | 10 | 12 |

| 3 | 6 | 9 | 12 | 15 | 18 |

| 4 | 8 | 12 | 16 | 20 | 24 |

| 5 | 10 | 15 | 20 | 25 | 30 |

| 6 | 12 | 18 | 24 | 30 | 36 |

And there are 4 12's in the table.

---
### [Problem 75](solutions/problem_075.js)
#### Difficulty: `Hard`

This problem was asked by Microsoft.

Given an array of numbers, find the length of the longest increasing subsequence in the array. The subsequence does not necessarily have to be contiguous.

For example, given the array `[0, 8, 4, 12, 2, 10, 6, 14, 1, 9, 5, 13, 3, 11, 7, 15]`, the longest increasing subsequence has length `6`: it is `0, 2, 6, 9, 11, 15`.

---
### [Problem 76](solutions/problem_076.js)
#### Difficulty: `Medium`

This problem was asked by Google.

You are given an N by M 2D matrix of lowercase letters. Determine the minimum number of columns that can be removed to ensure that each row is ordered from top to bottom lexicographically. That is, the letter at each column is lexicographically later as you go down each row. It does not matter whether each row itself is ordered lexicographically.

For example, given the following table:

```
cba
daf
ghi
```

This is not ordered because of the a in the center. We can remove the second column to make it ordered:

```
ca
df
gi
```

So your function should return 1, since we only needed to remove 1 column.

As another example, given the following table:

`abcdef`

Your function should return 0, since the rows are already ordered (there's only one row).

As another example, given the following table:

```
zyx
wvu
tsr
```

Your function should return `3`, since we would need to remove all the columns to order it.

---
### [Problem 77](solutions/problem_077.js)
#### Difficulty: `Easy`

This problem was asked by Snapchat.

Given a list of possibly overlapping intervals, return a new list of intervals where all overlapping intervals have been merged.

The input list is not necessarily ordered in any way.

For example, given `[(1, 3), (5, 8), (4, 10), (20, 25)]`, you should return `[(1, 3), (4, 10), (20, 25)]`.

---
### [Problem 78](solutions/problem_078.js)
#### Difficulty: `Medium`

This problem was asked by Google.

Given k sorted singly linked lists, write a function to merge all the lists into one sorted singly linked list.

---
### [Problem 79](solutions/problem_079.js)
#### Difficulty: `Medium`

This problem was asked by Facebook.

Given an array of integers, write a function to determine whether the array could become non-decreasing by modifying at most `1` element.

For example, given the array `[10, 5, 7]`, you should return `true`, since we can modify the 10 into a 1 to make the array non-decreasing.

Given the array `[10, 5, 1]`, you should return false, since we can't modify any one element to get a non-decreasing array.

---
### [Problem 80](solutions/problem_080.js)
#### Difficulty: `Easy`

This problem was asked by Google.

Given the root of a binary tree, return a deepest node. For example, in the following tree, return d.

```
    a
   / \
  b   c
 /
d
```

---
### [Problem 81](solutions/problem_081.js)
#### Difficulty: `Medium`

This problem was asked by Yelp.

Given a mapping of digits to letters (as in a phone number), and a digit string, return all possible letters the number could represent. You can assume each valid number in the mapping is a single digit.

For example if `{“2”: [“a”, “b”, “c”], 3: [“d”, “e”, “f”], …}` then `23` should return `[“ad”, “ae”, “af”, “bd”, “be”, “bf”, “cd”, “ce”, “cf"]`.

---
### [Problem 82](solutions/problem_082.js)
#### Difficulty: `Easy`

This problem was asked Microsoft.

Using a `read7()` method that returns `7` characters from a file, implement `readN(n)` which reads `n` characters.

For example, given a file with the content `“Hello world”`, three `read7()` returns `“Hello w”`, `“orld”` and then `“”`.

---
### [Problem 83](solutions/problem_083.js)
#### Difficulty: `Medium`

This problem was asked by Google.

Invert a binary tree.

For example, given the following tree:

```
   a
  / \
 b   c
/ \  /
d  e f
```

should become:

```
   a
  / \
  c  b
  \  / \
  f e  d
```

---
### [Problem 84](solutions/problem_084.js)
#### Difficulty: `Medium`

This problem was asked by Amazon.

Given a matrix of 1s and 0s, return the number of "islands" in the matrix. A 1 represents land and 0 represents water, so an island is a group of 1s that are neighboring whose perimeter is surrounded by water.

For example, this matrix has 4 islands.

```
1 0 0 0 0
0 0 1 1 0
0 1 1 0 0
0 0 0 0 0
1 1 0 0 1
1 1 0 0 1
```

---
### [Problem 85](solutions/problem_085.js)
#### Difficulty: `Easy`

This problem was asked by Facebook.

Given three 32-bit integers `x`, `y`, and `b`, return `x` if `b` is `1` and `y` if `b` is `0`, using only mathematical or bit operations. You can assume `b` can only be `1` or `0`.

---
### [Problem 86](solutions/problem_086.js)
#### Difficulty: `Medium`

This problem was asked by Google.

Given a string of parentheses, write a function to compute the minimum number of parentheses to be removed to make the string valid (i.e. each open parenthesis is eventually closed).

For example, given the string `()())()`, you should return `1`. Given the string `)(`, you should return `2`, since we must remove all of them.

---
### [Problem 87](solutions/problem_087.js)
#### Difficulty: `Hard`

This problem was asked by Uber.

A rule looks like this:

```
A NE B
```

This means this means point A is located northeast of point B.

```
A SW C
```

means that point A is southwest of C.

Given a list of rules, check if the sum of the rules validate. For example:

```
A N B
B NE C
C N A
```

does not validate, since A cannot be both north and south of C.

```
A NW B
A N B
```
is considered valid.

---
### [Problem 88](solutions/problem_088.js)
#### Difficulty: `Medium`

This question was asked by ContextLogic.

Implement division of two positive integers without using the division, multiplication, or modulus operators. Return the quotient as an integer, ignoring the remainder.

---
### [Problem 89](solutions/problem_089.js)
#### Difficulty: `Medium`

This problem was asked by LinkedIn.

Determine whether a tree is a valid binary search tree.

A binary search tree is a tree with two children, `left` and `right`, and satisfies the constraint that the key in the `left` child must be less than or equal to the `root` and the key in the `right` child must be greater than or equal to the `root`.

---
### [Problem 90](solutions/problem_090.js)
#### Difficulty: `Medium`

This question was asked by Google.

Given an integer `n` and a list of integers `l`, write a function that randomly generates a number from `0` to `n-1` that isn't in `l` (uniform).

---
### [Problem 91](solutions/problem_091.js)
#### Difficulty: `Easy`

This problem was asked by Dropbox.

What does the below code snippet print out? How can we fix the anonymous functions to behave as we'd expect?

```javascript
const functions = [];
for (var i = 0; i < 10; i++) {
  functions.push(() => i);
}

functions.forEach(f => console.log(f()));
```

---
### [Problem 92](solutions/problem_092.js)
#### Difficulty: `Hard`

This problem was asked by Airbnb.

We're given a hashmap associating each courseId key with a list of courseIds values, which represents that the prerequisites of courseId are courseIds. Return a sorted ordering of courses such that we can finish all courses.

Return null if there is no such ordering.

For example, given `{'CSC300': ['CSC100', 'CSC200'], 'CSC200': ['CSC100'], 'CSC100': []}`, should return `['CSC100', 'CSC200', 'CSCS300']`.

---
### [Problem 93](solutions/problem_093.js)
#### Difficulty: `Hard`

This problem was asked by Apple.

Given a tree, find the largest tree/subtree that is a BST.

Given a tree, return the size of the largest tree/subtree that is a BST.

---
### [Problem 94](solutions/problem_094.js)
#### Difficulty: `Easy`

This problem was asked by Google.

Given a binary tree of integers, find the maximum path sum between two nodes. The path must go through at least one node, and does not need to go through the root.

---
### [Problem 95](solutions/problem_095.js)
#### Difficulty: `Hard`

This problem was asked by Palantir.

Given a number represented by a list of digits, find the next greater permutation of a number, in terms of lexicographic ordering. If there is not greater permutation possible, return the permutation with the lowest value/ordering.

For example, the list `[1,2,3]` should return `[1,3,2]`. The list `[1,3,2]` should return `[2,1,3]`. The list `[3,2,1]` should return `[1,2,3]`.

Can you perform the operation without allocating extra memory (disregarding the input memory)?

---
### [Problem 96](solutions/problem_096.js)
#### Difficulty: `Easy`

This problem was asked by Microsoft.

Given a number in the form of a list of digits, return all possible permutations.

For example, given `[1,2,3]`, return `[[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]]`.

---
### [Problem 97](solutions/problem_097.js)
#### Difficulty: `Medium`

This problem was asked by Stripe.

Write a map implementation with a get function that lets you retrieve the value of a key at a particular time.

It should contain the following methods:

- `set(key, value, time)` sets key to value for t = time.
- `get(key, time)` gets the key at t = time.
The map should work like this. If we set a key at a particular time, it will maintain that value forever or until it gets set at a later time. In other words, when we get a key at a time, it should return the value that was set for that key set at the most recent time.

Consider the following examples:

```
d.set(1, 1, 0) # set key 1 to value 1 at time 0
d.set(1, 2, 2) # set key 1 to value 2 at time 2
d.get(1, 1) # get key 1 at time 1 should be 1
d.get(1, 3) # get key 1 at time 3 should be 2
```

```
d.set(1, 1, 5) # set key 1 to value 1 at time 5
d.get(1, 0) # get key 1 at time 0 should be null
d.get(1, 10) # get key 1 at time 10 should be 1
```

```
d.set(1, 1, 0) # set key 1 to value 1 at time 0
d.set(1, 2, 0) # set key 1 to value 2 at time 0
d.get(1, 0) # get key 1 at time 0 should be 2
```

---
### [Problem 98](solutions/problem_098.js)
#### Difficulty: `Easy`

This problem was asked by Coursera.

Given a 2D board of characters and a word, find if the word exists in the grid.

The word can be constructed from letters of sequentially adjacent cell, where "adjacent" cells are those horizontally or vertically neighboring. The same letter cell may not be used more than once.

For example, given the following board:
```
[
  ['A','B','C','E'],
  ['S','F','C','S'],
  ['A','D','E','E']
]
```

- `exists(board, "ABCCED")` returns `true`
- `exists(board, "SEE")` returns `true`
- `exists(board, "ABCB")` returns `false`

---
### [Problem 99](solutions/problem_099.js)
#### Difficulty: `Medium`

This problem was asked by Microsoft.

Given an unsorted array of integers, find the length of the longest consecutive elements sequence.

For example, given `[100, 4, 200, 1, 3, 2]`, the longest consecutive element sequence is `[1, 2, 3, 4]`. Return its length: `4`.

Your algorithm should run in `O(n)` complexity.

---
### [Problem 100](solutions/problem_100.js)
#### Difficulty: `Easy`

This problem was asked by Google.

You are in an infinite 2D grid where you can move in any of the 8 directions:

```
(x,y) to
   (x+1, y),
   (x - 1, y),
   (x, y+1),
   (x, y-1),
   (x-1, y-1),
   (x+1,y+1),
   (x-1,y+1),
   (x+1,y-1)
```

You are given a sequence of points and the order in which you need to cover the points. Give the minimum number of steps in which you can achieve it. You start from the first point.

Example:

```
Input: [(0, 0), (1, 1), (1, 2)]
Output: 2
```

It takes `1` step to move from `(0, 0)` to `(1, 1)`. It takes one more step to move from `(1, 1)` to `(1, 2)`.

---
### [Problem 101](solutions/problem_101.js)
#### Difficulty: `Easy`

This problem was asked by Alibaba.

Given an even number (greater than `2`), return two prime numbers whose sum will be equal to the given number.

A solution will always exist. See [Goldbach’s conjecture](https://en.wikipedia.org/wiki/Goldbach%27s_conjecture).

Example:

```
Input: 4
Output: 2 + 2 = 4
```

If there are more than one solution possible, return the lexicographically smaller solution.

If `[a, b]` is one solution with `a <= b`, and `[c, d]` is another solution with `c <= d`, then

`[a, b] < [c, d]`
If `a < c OR a==c AND b < d`.

---
### [Problem 102](solutions/problem_102.js)
#### Difficulty: `Medium`

This problem was asked by Lyft.

Given a list of integers and a number `K`, return which contiguous elements of the list sum to `K`.

For example, if the list is `[1, 2, 3, 4, 5]` and `K` is `9`, then it should return `[2, 3, 4]`, since `2 + 3 + 4 = 9`.

---
### [Problem 103](solutions/problem_103.js)
#### Difficulty: `Medium`

This problem was asked by Square.

Given a string and a set of characters, return the shortest substring containing all the characters in the set.

For example, given the string "figehaeci" and the set of characters {a, e, i}, you should return "aeci".

If there is no substring containing all the characters in the set, return null.

---
### [Problem 104](solutions/problem_104.js)
#### Difficulty: `Easy`

This problem was asked by Google.

Determine whether a doubly linked list is a palindrome. What if it’s singly linked?

For example, `1 -> 4 -> 3 -> 4 -> 1` returns `True` while `1 -> 4` returns `False`.

---
### [Problem 105](solutions/problem_105.js)
#### Difficulty: `Easy`

This problem was asked by Facebook.

Given a function `f`, and `N` return a debounced `f` of `N` milliseconds.

That is, as long as the debounced `f` continues to be invoked, `f` itself will not be called for `N` milliseconds.

---
### [Problem 106](solutions/problem_106.js)
#### Difficulty: `Medium`

This problem was asked by Pinterest.

Given an integer list where each number represents the number of hops you can make, determine whether you can reach to the last index starting at index `0`.

For example, `[2, 0, 1, 0]` returns `True` while `[1, 1, 0, 1]` returns `False`.

---
### [Problem 107](solutions/problem_107.js)
#### Difficulty: `Easy`

This problem was asked by Microsoft.

Print the nodes in a binary tree level-wise. For example, the following should print `1, 2, 3, 4, 5`.

```
  1
 / \
2   3
   / \
  4   5
```

---
### [Problem 108](solutions/problem_108.js)
#### Difficulty: `Easy`

This problem was asked by Google.

Given two strings `A` and `B`, return whether or not `A` can be shifted some number of times to get `B`.

For example, if `A` is `abcde` and `B` is `cdeab`, return `true`. If `A` is `abc` and `B` is `acb`, return `false`.

---
### [Problem 109](solutions/problem_109.js)
#### Difficulty: `Medium`

This problem was asked by Cisco.

Given an unsigned 8-bit integer, swap its even and odd bits. The 1st and 2nd bit should be swapped, the 3rd and 4th bit should be swapped, and so on.

For example, `10101010` should be `01010101`. `11100010` should be `11010001`.

Bonus: Can you do this in one line?

---
### [Problem 110](solutions/problem_110.js)
#### Difficulty: `Medium`

This problem was asked by Facebook.

Given a binary tree, return all paths from the root to leaves.

For example, given the tree:

```
   1
  / \
 2   3
    / \
   4   5
```

Return `[[1, 2], [1, 3, 4], [1, 3, 5]]`.

---
### [Problem 111](solutions/problem_111.js)
#### Difficulty: `Hard`

This problem was asked by Google.

Given a word `W` and a string `S`, find all starting indices in `S` which are anagrams of `W`.

For example, given that `W` is `ab`, and `S` is `abxaba`, return `0`, `3`, and `4`.

---
### [Problem 112](solutions/problem_112.js)
#### Difficulty: `Hard`

This problem was asked by Twitter.

Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree. Assume that each node in the tree also has a pointer to its parent.

According to the definition of [LCA](https://en.wikipedia.org/wiki/Lowest_common_ancestor) on Wikipedia: “The lowest common ancestor is defined between two nodes `v` and `w` as the lowest node in `T` that has both `v` and `w` as descendants (where we allow a node to be a descendant of itself).”

---
### [Problem 113](solutions/problem_113.js)
#### Difficulty: `Medium`

This problem was asked by Google.

Given a string of words delimited by spaces, reverse the words in string. For example, given `hello world here`, return `here world hello`

Follow-up: given a mutable string representation, can you perform this operation in-place?

---
### [Problem 114](solutions/problem_114.js)
#### Difficulty: `Hard`

This problem was asked by Facebook.

Given a string and a set of delimiters, reverse the words in the string while maintaining the relative order of the delimiters. For example, given `hello/world:here`, return `here/world:hello`

Follow-up: Does your solution work for the following cases: `hello/world:here/`, `hello//world:here`

---
### [Problem 115](solutions/problem_115.js)
#### Difficulty: `Hard`

This problem was asked by Google.

Given two non-empty binary trees `s` and `t`, check whether tree `t` has exactly the same structure and node values with a subtree of `s`. A subtree of `s` is a tree consists of a node in `s` and all of this node's descendants. The tree `s` could also be considered as a subtree of itself.

---
### [Problem 116](solutions/problem_116.js)
#### Difficulty: `Medium`

This problem was asked by Jane Street.

Generate a finite, but an arbitrarily large binary tree quickly in `O(1)`.

That is, `generate()` should return a tree whose size is unbounded but finite.

---
### [Problem 117](solutions/problem_117.js)
#### Difficulty: `Easy`

This problem was asked by Facebook.

Given a binary tree, return the level of the tree with minimum sum.

---
### [Problem 118](solutions/problem_118.js)
#### Difficulty: `Easy`

This problem was asked by Google.

Given a sorted list of integers, square the elements and give the output in sorted order.

For example, given `[-9, -2, 0, 2, 3]`, return `[0, 4, 4, 9, 81]`.

---
### [Problem 119](solutions/problem_119.js)
#### Difficulty: `Medium`

This problem was asked by Google.

Given a set of closed intervals, find the smallest set of numbers that covers all the intervals. If there are multiple smallest sets, return any of them.

For example, given the intervals `[0, 3], [2, 6], [3, 4], [6, 9]`, one set of numbers that covers all these intervals is `{3, 6}`.

---
### [Problem 120](solutions/problem_120.js)
#### Difficulty: `Medium`

This problem was asked by Microsoft.

Implement the singleton pattern with a twist. First, instead of storing one instance, store two instances. And in every even call of `getInstance()`, return the first instance and in every odd call of `getInstance()`, return the second instance.

---
### [Problem 121](solutions/problem_121.js)
#### Difficulty: `Hard`

This problem was asked by Google.

Given a string which we can delete at most `k`, return whether you can make a palindrome.

For example, given `waterrfetawx` and a `k` of `2`, you could delete `f` and `x` to get `waterretaw`.

---
### [Problem 122](solutions/problem_122.js)
#### Difficulty: `Medium`

This question was asked by Zillow.

You are given a 2-d matrix where each cell represents number of coins in that cell. Assuming we start at `matrix[0][0]`, and can only move `right` or `down`, find the maximum number of coins you can collect by the bottom right corner.

For example, in this matrix

```
0 3 1 1
2 0 0 4
1 5 3 1
```

The most we can collect is `0 + 2 + 1 + 5 + 3 + 1 = 12` coins.

---
### [Problem 123](solutions/problem_123.js)
#### Difficulty: `Hard`

This problem was asked by LinkedIn.

Given a string, return whether it represents a number. Here are the different kinds of numbers:

- "10", a positive integer
- "-10", a negative integer
- "10.1", a positive real number
- "-10.1", a negative real number
- "1e5", a number in scientific notation

And here are examples of non-numbers:

- "a"
- "x 1"
- "a -2"
- "-"

---
### [Problem 124](solutions/problem_124.js)
#### Difficulty: `Easy`

This problem was asked by Microsoft.

You have `n` fair coins and you flip them all at the same time. Any that come up tails you set aside. The ones that come up heads you flip again. How many rounds do you expect to play before only one coin remains?

Write a function that, given `n`, returns the number of rounds you'd expect to play until one coin remains.

---
### [Problem 125](solutions/problem_125.js)
#### Difficulty: `Easy`

This problem was asked by Google.

Given the root of a binary search tree, and a target `K`, return two nodes in the tree whose sum equals `K`.

For example, given the following tree and `K` of `20`

```
    10
   /   \
 5      15
       /  \
     11    15
```

Return the nodes `5` and `15`.

---
### [Problem 126](solutions/problem_126.js)
#### Difficulty: `Medium`

This problem was asked by Facebook.

Write a function that rotates a list by `k` elements. For example, `[1, 2, 3, 4, 5, 6]` rotated by two becomes `[3, 4, 5, 6, 1, 2]`. Try solving this without creating a copy of the list. How many swap or move operations do you need?

---
### [Problem 127](solutions/problem_127.js)
#### Difficulty: `Easy`

This problem was asked by Microsoft.

Let's represent an integer in a linked list format by having each node represent a digit in the number. The nodes make up the number in reversed order.

For example, the following linked list:

`1 -> 2 -> 3 -> 4 -> 5`
is the number `54321`.

Given two linked lists in this format, return their sum in the same linked list format.

For example, given

```
9 -> 9
5 -> 2 -> 1
```

return `224` (`99 + 125`) as:

`4 -> 2 -> 2`

---
### [Problem 128](solutions/problem_128.js)
#### Difficulty: `Medium`


The Tower of Hanoi is a puzzle game with three rods and `n` disks, each a different size.

All the disks start off on the first rod in a stack. They are ordered by size, with the largest disk on the bottom and the smallest one at the top.

The goal of this puzzle is to move all the disks from the first rod to the last rod while following these rules:

- You can only move one disk at a time.
- A move consists of taking the uppermost disk from one of the stacks and placing it on top of another stack.
- You cannot place a larger disk on top of a smaller disk.

Write a function that prints out all the steps necessary to complete the Tower of Hanoi. You should assume that the rods are numbered, with the first rod being `1`, the second (auxiliary) rod being `2`, and the last (goal) rod being `3`.

For example, with `n = 3`, we can do this in `7` moves:

```
Move 1 to 3
Move 1 to 2
Move 3 to 2
Move 1 to 3
Move 2 to 1
Move 2 to 3
Move 1 to 3
```

---
### [Problem 129](solutions/problem_129.js)
#### Difficulty: `Medium`

Given a real number `n`, find the square root of `n`. For example, given `n` = `9`, return `3`.

---
### [Problem 130](solutions/problem_130.js)
#### Difficulty: `Medium`

This problem was asked by Facebook.

Given an array of numbers representing the stock prices of a company in chronological order and an integer `k`, return the maximum profit you can make from `k` buys and sells. You must buy the stock before you can sell it, and you must sell the stock before you can buy it again.

For example, given `k` = `2` and the array `[5, 2, 4, 0, 1]`, you should return `3`.

---
### [Problem 131](solutions/problem_131.js)
#### Difficulty: `Medium`

This question was asked by Snapchat.

Given the head to a singly linked list, where each node also has a “random” pointer that points to anywhere in the linked list, deep clone the list.

---
### [Problem 132](solutions/problem_132.js)
#### Difficulty: `Easy`

This question was asked by Riot Games.

Design and implement a `HitCounter` class that keeps track of requests (or hits). It should support the following operations:

- `record(timestamp)`: records a hit that happened at timestamp
- `total()`: returns the total number of hits recorded
- `range(lower, upper)`: returns the number of hits that occurred between timestamps lower and upper (inclusive)

Follow-up: What if our system has limited memory?

---
### [Problem 133](solutions/problem_133.js)
#### Difficulty: `Medium`

This problem was asked by Amazon.

Given a `node` in a binary search tree, return the next bigger element, also known as the inorder successor.

For example, the inorder successor of `22` is `30`.

```
   10
  /  \
 5    30
     /  \
   22    35
```

You can assume each node has a parent pointer.

---
### [Problem 134](solutions/problem_134.js)
#### Difficulty: `Easy`

This problem was asked by Facebook.

You have a large array with most of the elements as zero.

Use a more space-efficient data structure, `SparseArray`, that implements the same interface:

- `init(arr, size)`: initialize with the original large array and size.
- `set(i, val)`: updates index at `i` with `val`.
- `get(i)`: gets the value at index `i`.

---
### [Problem 135](solutions/problem_135.js)
#### Difficulty: `Easy`

This question was asked by Apple.

Given a binary tree, find a minimum path sum from root to a leaf.

For example, the minimum path in this tree is `[10, 5, 1, -1]`, which has sum `15`.

```
  10
 /  \
5    5
 \     \
   2    1
       /
     -1
```

---
### [Problem 136](solutions/problem_136.js)
#### Difficulty: `Medium`

This question was asked by Google.

Given an `N` by `M` matrix consisting only of `1`'s and `0`'s, find the largest rectangle containing only `1`'s and return its area.

For example, given the following matrix:

```
[[1, 0, 0, 0],
 [1, 0, 1, 1],
 [1, 0, 1, 1],
 [0, 1, 0, 0]]
```

Return `4`.

---
### [Problem 137](solutions/problem_137.js)
#### Difficulty: `Medium`

This problem was asked by Amazon.

Implement a bit array.

A bit array is a space efficient array that holds a value of `1` or `0` at each index.

- `init(size)`: initialize the array with size
- `set(i, val)`: updates index at `i` with `val` where `val` is either `1` or `0`.
- `get(i)`: gets the value at index `i`.

---
### [Problem 138](solutions/problem_138.js)
#### Difficulty: `Hard`

This problem was asked by Google.

Find the minimum number of coins required to make `n` cents.

You can use standard American denominations, that is, `1¢`, `5¢`, `10¢`, and `25¢`.

For example, given `n = 16`, return `3` since we can make it with a `10¢`, a `5¢`, and a `1¢`.

---
### [Problem 139](solutions/problem_139.js)
#### Difficulty: `Medium`

This problem was asked by Google.

Given an iterator with methods `next()` and `hasNext()`, create a wrapper iterator, `PeekableInterface`, which also implements `peek()`. peek shows the next element that would be returned on `next()`.

Here is the interface:

```
class PeekableInterface(object):
    def __init__(self, iterator):
        pass

    def peek(self):
        pass

    def next(self):
        pass

    def hasNext(self):
        pass
```

---
### [Problem 140](solutions/problem_140.js)
#### Difficulty: `Medium`

This problem was asked by Facebook.

Given an array of integers in which two elements appear exactly once and all other elements appear exactly twice, find the two elements that appear only once.

For example, given the array `[2, 4, 6, 8, 10, 2, 6, 10]`, return `4` and `8`. The order does not matter.

Follow-up: Can you do this in linear time and constant space?

---
### [Problem 141](solutions/problem_141.js)
#### Difficulty: `Hard`

This problem was asked by Microsoft.

Implement 3 stacks using a single list:

```
class Stack:
    def __init__(self):
        self.list = []

    def pop(self, stack_number):
        pass

    def push(self, item, stack_number):
        pass
```

---
### [Problem 142](solutions/problem_142.js)
#### Difficulty: `Hard`

This problem was asked by Google.

You're given a string consisting solely of `(`, `)`, and `*.` `*` can represent either a `(`, `)`, or an empty string. Determine whether the parentheses are balanced.

For example, `(()*` and `(*)` are balanced. `)*(` is not balanced.

---
### [Problem 143](solutions/problem_143.js)
#### Difficulty: `Medium`

This problem was asked by Amazon.

Given a pivot `x`, and a list `lst`, partition the list into three parts.

The first part contains all elements in `lst` that are less than `x`
The second part contains all elements in `lst` that are equal to `x`
The third part contains all elements in `lst` that are larger than `x`
Ordering within a part can be arbitrary.

For example, given `x = 10` and `lst = [9, 12, 3, 5, 14, 10, 10]`, one partition may be `[9, 3, 5, 10, 10, 12, 14]`.

---
### [Problem 144](solutions/problem_144.js)
#### Difficulty: `Medium`

This problem was asked by Google.

Given an array of numbers and an index `i`, return the index of the nearest larger number of the number at index `i`, where distance is measured in array indices.

For example, given `[4, 1, 3, 5, 6]` and index `0`, you should return `3`.

If two distances to larger numbers are the equal, then return any one of them. If the array at `i` doesn't have a nearest larger integer, then return null.

Follow-up: If you can preprocess the array, can you do this in constant time?

---
### [Problem 145](solutions/problem_145.js)
#### Difficulty: `Easy`

This problem was asked by Google.

Given the head of a singly linked list, swap every two nodes and return its head.

For example, given `1 -> 2 -> 3 -> 4`, return `2 -> 1 -> 4 -> 3`.

---
### [Problem 146](solutions/problem_146.js)
#### Difficulty: `Medium`

This question was asked by BufferBox.

Given a binary tree where all nodes are either `0` or `1`, prune the tree so that subtrees containing all `0s` are removed.

For example, given the following tree:

```
   0
  / \
 1   0
    / \
   1   0
  / \
 0   0
```
should be pruned to:

```
   0
  / \
 1   0
    /
   1
```

We do not remove the tree at the root or its left child because it still has a `1` as a descendant.

---
### [Problem 147](solutions/problem_147.js)
#### Difficulty: `Hard`

Given a list, sort it using this method: `reverse(lst, i, j)`, which reverses `lst` from `i` to `j`.

---
### [Problem 148](solutions/problem_148.js)
#### Difficulty: `Medium`

This problem was asked by Apple.

[Gray code](https://en.wikipedia.org/wiki/Gray_code) is a binary code where each successive value differ in only one bit, as well as when wrapping around. Gray code is common in hardware so that we don't see temporary spurious values during transitions.

Given a number of bits `n`, generate a possible gray code for it.

For example, for `n = 2`, one gray code would be `[00, 01, 11, 10]`.

---
### [Problem 149](solutions/problem_149.js)
#### Difficulty: `Hard`

This problem was asked by Goldman Sachs.

Given a list of numbers `L`, implement a method `sum(i, j)` which returns the sum from the sublist `L[i:j]` (including `i`, excluding `j`).

For example, given `L = [1, 2, 3, 4, 5]`, `sum(1, 3)` should return `sum([2, 3])`, which is `5`.

You can assume that you can do some pre-processing. `sum()` should be optimized over the pre-processing step.

---
### [Problem 150](solutions/problem_150.js)
#### Difficulty: `Hard`

This problem was asked by LinkedIn.

Given a list of points, a central point, and an integer `k`, find the nearest `k` points from the central point.

For example, given the list of points `[(0, 0), (5, 4), (3, 1)]`, the central point `(1, 2)`, and `k = 2`, return `[(0, 0), (3, 1)]`.

---
### [Problem 151](solutions/problem_151.js)
#### Difficulty: `Medium`

Given a 2-D matrix representing an image, a location of a pixel in the screen and a color `C`, replace the color of the given pixel and all adjacent same colored pixels with `C`.

For example, given the following matrix, and location pixel of `(2, 2)`, and '`G`' for green:

```
B B W
W W W
W W W
B B B
```
Becomes
```
B B G
G G G
G G G
B B B
```

---
### [Problem 152](solutions/problem_152.js)
#### Difficulty: `Medium`

This problem was asked by Triplebyte.

You are given `n` numbers as well as `n` probabilities that sum up to `1`. Write a function to generate one of the numbers with its corresponding probability.

For example, given the numbers `[1, 2, 3, 4]` and probabilities `[0.1, 0.5, 0.2, 0.2]`, your function should return `1` 10% of the time, `2` 50% of the time, and `3` and `4` 20% of the time.

You can generate random numbers between `0` and `1` uniformly.

---
### [Problem 153](solutions/problem_153.js)
#### Difficulty: `Hard`

Find an efficient algorithm to find the smallest distance (measured in number of words) between any two given words in a string.

For example, given words "`hello`", and "`world`" and a text content of "`dog cat hello cat dog dog hello cat world`", return `1` because there's only one word "`cat`" in between the two words.

---
### [Problem 154](solutions/problem_154.js)
#### Difficulty: `Easy`

This problem was asked by Amazon.

Implement a stack API using only a heap. A stack implements the following methods:

- `push(item)` - which adds an element to the stack
- `pop()`-  which removes and returns the most recently added element (or throws an error if there is nothing on the stack)

Recall that a heap has the following operations:

- `push(item)` - which adds a new key to the heap
- `pop()` - which removes and returns the max value of the heap

---
### [Problem 155](solutions/problem_155.js)
#### Difficulty: `Medium`

This problem was asked by MongoDB.

Given a list of elements, find the majority element, which appears more than half the time (`> floor(len(lst) / 2.0`)).

You can assume that such element exists.

For example, given `[1, 2, 1, 1, 3, 4, 0]`, return `1`.

---
### [Problem 156](solutions/problem_156.js)
#### Difficulty: `Medium`

This problem was asked by Facebook.

Given a positive integer `n`, find the smallest number of squared integers which sum to `n`.

For example, given `n = 13`, return `2` since `13 = 3^2 + 2^2 = 9 + 4`.

Given `n = 27`, return `3` since `27 = 3^2 + 3^2 + 3^2 = 9 + 9 + 9`.

Given `n = 20`, return `2` since `20 = 4^2 + 2^2 = 16 + 4`.

---
### [Problem 157](solutions/problem_157.js)
#### Difficulty: `Easy`

This problem was asked by Amazon.

Given a string, determine whether any permutation of it is a palindrome.

For example, `carrace` should return `true`, since it can be rearranged to form `racecar`, which is a palindrome. `daily` should return `false`, since there's no rearrangement that can form a palindrome.

---
### [Problem 158](solutions/problem_158.js)
#### Difficulty: `Medium`

This problem was asked by Slack.

You are given an `N` by `M` matrix of `0`s and `1`s. Starting from the top left corner, how many ways are there to reach the bottom right corner?

You can only move right and down. `0` represents an empty space while `1` represents a wall you cannot walk through.

For example, given the following matrix:

```
[[0, 0, 1],
 [0, 0, 1],
 [1, 0, 0]]
 ```

Return `2`, as there are only two ways to get to the bottom right:

- `Right, down, down, right`
- `Down, right, down, right`

The top left corner and bottom right corner will always be `0`.

---
### [Problem 159](solutions/problem_159.js)
#### Difficulty: `Easy`

This problem was asked by Google.

Given a string, return the first recurring character in it, or `null` if there is no recurring character.

For example, given the string `acbbac`, return `b`. Given the string `abcdef`, return `null`.

---
### [Problem 160](solutions/problem_160.js)
#### Difficulty: `Hard`

This problem was asked by Uber.

Given a tree where each edge has a weight, compute the length of the longest path in the tree.

For example, given the following tree:

```
   a
  /|\
 b c d
    / \
   e   f
  / \
 g   h
```

and the weights: `a-b: 3, a-c: 5, a-d: 8, d-e: 2, d-f: 4, e-g: 1, e-h: 1`, the longest path would be `c -> a -> d -> f`, with a length of `17`.

The path does not have to pass through the `root`, and each node can have any amount of children.

---
### [Problem 161](solutions/problem_161.js)
#### Difficulty: `Easy`

This problem was asked by Facebook.

Given a 32-bit integer, return the number with its bits reversed.

For example, given the binary number `1111 0000 1111 0000 1111 0000 1111 0000`, return `0000 1111 0000 1111 0000 1111 0000 1111`.

---
### [Problem 162](solutions/problem_162.js)
#### Difficulty: `Medium`

This problem was asked by Square.

Given a list of words, return the shortest unique prefix of each word. For example, given the list:

- dog
- cat
- apple
- apricot
- fish

Return the list:

- d
- c
- app
- apr
- f

---
### [Problem 163](solutions/problem_163.js)
#### Difficulty: `Hard`

This problem was asked by Jane Street.

Given an arithmetic expression in [Reverse Polish Notation](https://en.wikipedia.org/wiki/Reverse_Polish_notation), write a program to evaluate it.

The expression is given as a list of numbers and operands. For example: `[5, 3, '+']` should return `5 + 3 = 8`.

For example, `[15, 7, 1, 1, '+', '-', '/', 3, '*', 2, 1, 1, '+', '+', '-']` should return `5`, since it is equivalent to `((15 / (7 - (1 + 1))) * 3) - (2 + (1 + 1)) = 5`.

You can assume the given expression is always valid.

---
### [Problem 164](solutions/problem_164.js)
#### Difficulty: `Medium`

This problem was asked by Google.

You are given an array of length `n + 1` whose elements belong to the set `{1, 2, ..., n}`. By the pigeonhole principle, there must be a duplicate. Find it in linear time and space.

---
### [Problem 165](solutions/problem_165.js)
#### Difficulty: `Medium`

This problem was asked by Google.

Given an array of integers, return a new array where each element in the new array is the number of smaller elements to the right of that element in the original input array.

For example, given the array `[3, 4, 9, 6, 1]`, return `[1, 1, 2, 1, 0]`, since:

- There is `1` smaller element to the right of `3`
- There is `1` smaller element to the right of `4`
- There are `2` smaller elements to the right of `9`
- There is `1` smaller element to the right of `6`
- There are no smaller elements to the right of `1`

---
### [Problem 166](solutions/problem_166.js)
#### Difficulty: `Medium`

This problem was asked by Uber.

Implement a 2D iterator class. It will be initialized with an array of arrays, and should implement the following methods:

- `next()`: returns the next element in the array of arrays. If there are no more elements, raise an exception.
- `has_next()`: returns whether or not the iterator still has elements left.

For example, given the input `[[1, 2], [3], [], [4, 5, 6]]`, calling `next()` repeatedly should output `1, 2, 3, 4, 5, 6`.

Do not use flatten or otherwise clone the arrays. Some of the arrays can be empty.

---
### [Problem 167](solutions/problem_167.js)
#### Difficulty: `Hard`

This problem was asked by Airbnb.

Given a list of words, find all pairs of unique indices such that the concatenation of the two words is a palindrome.

For example, given the list `["code", "edoc", "da", "d"]`, return `[[0, 1], [1, 0], [2, 3]]`.

---
### [Problem 168](solutions/problem_168.js)
#### Difficulty: `Medium`

This problem was asked by Facebook.

Given an N by N matrix, rotate it by 90 degrees clockwise.

For example, given the following matrix:

```
[[1, 2, 3],
 [4, 5, 6],
 [7, 8, 9]]
```

you should return:

```
[[7, 4, 1],
 [8, 5, 2],
 [9, 6, 3]]
```

Follow-up: What if you couldn't use any extra space?

---
### [Problem 169](solutions/problem_169.js)
#### Difficulty: `Medium`

This problem was asked by Google.

Given a linked list, sort it in `O(n log n)` time and constant space.

For example, the linked list `4 -> 1 -> -3 -> 99` should become `-3 -> 1 -> 4 -> 99`.

---
### [Problem 170](solutions/problem_170.js)
#### Difficulty: `Medium`

This problem was asked by Facebook.

Given a `start` word, an `end` word, and a dictionary of valid words, find the shortest transformation sequence from start to end such that only one letter is changed at each step of the sequence, and each transformed word exists in the dictionary. If there is no possible transformation, return `null`. Each word in the dictionary have the same length as start and end and is lowercase.

For example, given start = "dog", end = "cat", and dictionary = `{"dot", "dop", "dat", "cat"}`, return `["dog", "dot", "dat", "cat"]`.

Given `start = "dog"`, `end = "cat"`, and `dictionary = {"dot", "tod", "dat", "dar"}`, return `null` as there is no possible transformation from `dog` to `cat`.

---
### [Problem 171](solutions/problem_171.js)
#### Difficulty: `Easy`

This problem was asked by Amazon.

You are given a list of data entries that represent entries and exits of groups of people into a building. An entry looks like this:

`{"timestamp": 1526579928, count: 3, "type": "enter"}`

This means `3` people entered the building. An exit looks like this:

`{"timestamp": 1526580382, count: 2, "type": "exit"}`

This means that `2` people exited the building. timestamp is in Unix time.

Find the busiest period in the building, that is, the time with the most people in the building. Return it as a pair of (start, end) timestamps. You can assume the building always starts off and ends up empty, i.e. with 0 people inside.

---
### [Problem 172](solutions/problem_172.js)
#### Difficulty: `Medium`

This problem was asked by Dropbox.

Given a string `s` and a list of words `words`, where each word is the same length, find all starting indices of substrings in `s` that is a concatenation of every word in `words` exactly once.

For example, given `s = dogcatcatcodecatdog` and `words = ["cat", "dog"]`, return `[0, 13]`, since `dogcat` starts at index `0` and `catdog` starts at index `13`.

Given `s = barfoobazbitbyte` and `words = ["dog", "cat"]`, return `[]` since there are no substrings composed of `dog` and `cat` in `s`.

The order of the indices does not matter.

---
### [Problem 173](solutions/problem_173.js)
#### Difficulty: `Easy`

This problem was asked by Stripe.

Write a function to flatten a nested dictionary. Namespace the keys with a period.

For example, given the following dictionary:

```
{
    "key": 3,
    "foo": {
        "a": 5,
        "bar": {
            "baz": 8
        }
    }
}
```

it should become:

```
{
    "key": 3,
    "foo.a": 5,
    "foo.bar.baz": 8
}
```

You can assume keys do not contain dots in them, i.e. no clobbering will occur.

---
### [Problem 174](solutions/problem_174.js)
#### Difficulty: `Medium`

This problem was asked by Microsoft.

Describe and give an example of each of the following types of polymorphism:

- Ad-hoc polymorphism
- Parametric polymorphism
- Subtype polymorphism

---
### [Problem 175](solutions/problem_175.js)
#### Difficulty: `Easy`

This problem was asked by Google.

You are given a starting state start, a list of transition probabilities for a Markov chain, and a number of steps `num_steps`. Run the Markov chain starting from start for `num_steps` and compute the number of times we visited each state.

For example, given the starting state `a`, number of steps `5000`, and the following transition probabilities:

```
[
  ['a', 'a', 0.9],
  ['a', 'b', 0.075],
  ['a', 'c', 0.025],
  ['b', 'a', 0.15],
  ['b', 'b', 0.8],
  ['b', 'c', 0.05],
  ['c', 'a', 0.25],
  ['c', 'b', 0.25],
  ['c', 'c', 0.5]
]
```

One instance of running this Markov chain might produce `{ 'a': 3012, 'b': 1656, 'c': 332 }`.

---
### [Problem 176](solutions/problem_176.js)
#### Difficulty: `Easy`

This problem was asked by Bloomberg.

Determine whether there exists a one-to-one character mapping from one string `s1` to another `s2`.

For example, given `s1 = abc` and `s2 = bcd`, return `true` since we can map `a` to `b`, `b` to `c`, and `c` to `d`.

Given `s1 = foo` and `s2 = bar`, return `false` since the `o` cannot map to two characters.

---
### [Problem 177](solutions/problem_177.js)
#### Difficulty: `Easy`

This problem was asked by Airbnb.

Given a linked list and a positive integer `k`, rotate the list to the right by `k` places.

For example, given the linked list `7 -> 7 -> 3 -> 5` and `k = 2`, it should become `3 -> 5 -> 7 -> 7`.

Given the linked list `1 -> 2 -> 3 -> 4 -> 5` and `k = 3`, it should become `3 -> 4 -> 5 -> 1 -> 2`.

---
### [Problem 178](solutions/problem_178.js)
#### Difficulty: `Hard`

This problem was asked by Two Sigma.

Alice wants to join her school's Probability Student Club. Membership dues are computed via one of two simple probabilistic games.

The first game: roll a die repeatedly. Stop rolling once you get a five followed by a six. Your number of rolls is the amount you pay, in dollars.

The second game: same, except that the stopping condition is a five followed by a five.

Which of the two games should Alice elect to play? Does it even matter? Write a program to simulate the two games and calculate their expected value.

---
### [Problem 179](solutions/problem_179.js)
#### Difficulty: `Medium`

This problem was asked by Google.

Given the sequence of keys visited by a postorder traversal of a binary search tree, reconstruct the tree.

For example, given the sequence `2, 4, 3, 8, 7, 5`, you should construct the following tree:

```
    5
   / \
  3   7
 / \   \
2   4   8
```

---
### [Problem 180](solutions/problem_180.js)
#### Difficulty: `Medium`

This problem was asked by Google.

Given a stack of `N` elements, interleave the first half of the stack with the second half reversed using only one other queue. This should be done in-place.

Recall that you can only `push(item)` or `pop()` from a stack, and `enqueue(item)` or `dequeue()` from a queue.

For example, if the stack is `[1, 2, 3, 4, 5]`, it should become `[1, 5, 2, 4, 3]`. If the stack is `[1, 2, 3, 4]`, it should become [`1, 4, 2, 3]`.

Hint: Try working backwards from the end state.

---
### [Problem 181](solutions/problem_181.js)
#### Difficulty: `Medium`

This problem was asked by Google.

Given a string, split it into as few strings as possible such that each string is a palindrome.

For example, given the input string `racecarannakayak`, return `["racecar", "anna", "kayak"]`.

Given the input string `abc`, return `["a", "b", "c"]`.

---
### [Problem 182](solutions/problem_182.js)
#### Difficulty: `Medium`

This problem was asked by Facebook.

A graph is minimally-connected if it is connected and there is no edge that can be removed while still leaving the graph connected. For example, any binary tree is minimally-connected.

Given an undirected graph, check if the graph is minimally-connected. You can choose to represent the graph as either an adjacency matrix or adjacency list.

---
### [Problem 183](solutions/problem_183.js)
#### Difficulty: `Hard`

This problem was asked by Twitch.

Describe what happens when you type a URL into your browser and press Enter.

---
### [Problem 184](solutions/problem_184.js)
#### Difficulty: `Easy`

This problem was asked by Amazon.

Given n numbers, find the greatest common denominator between them.

For example, given the numbers `[42, 56, 14]`, return `14`.

---
### [Problem 185](solutions/problem_185.js)
#### Difficulty: `Easy`

This problem was asked by Google.

Given two rectangles on a 2D graph, return the area of their intersection. If the rectangles don't intersect, return `0`.

For example, given the following rectangles:

```
{
    "top_left": [1, 4],
    "dimensions": [3, 3] # width, height
}
```

and

```
{
    "top_left": [0, 5],
    "dimensions": [4, 3] # width, height
}
```

return `6`.

---
### [Problem 186](solutions/problem_186.js)
#### Difficulty: `Hard`

This problem was asked by Microsoft.

Given an array of positive integers, divide the array into two subsets such that the difference between the sum of the subsets is as small as possible.

For example, given `[5, 10, 15, 20, 25]`, return the sets `{10, 25}` and `{5, 15, 20}`, which has a difference of `5`, which is the smallest possible difference.

---
### [Problem 187](solutions/problem_187.js)
#### Difficulty: `Easy`

This problem was asked by Google.

You are given given a list of rectangles represented by min and max x- and y-coordinates. Compute whether or not a pair of rectangles overlap each other. If one rectangle completely covers another, it is considered overlapping.

For example, given the following rectangles:

```
{
    "top_left": (1, 4),
    "dimensions": (3, 3) # width, height
},
{
    "top_left": (-1, 3),
    "dimensions": (2, 1)
},
{
    "top_left": (0, 5),
    "dimensions": (4, 3)
}
```

return `true` as the first and third rectangle overlap each other.

---
### [Problem 188](solutions/problem_188.js)
#### Difficulty: `Easy`

This problem was asked by Google.

What will this code print out?

```
def make_functions():
    flist = []

    for i in [1, 2, 3]:
        def print_i():
            print(i)
        flist.append(print_i)

    return flist

functions = make_functions()
for f in functions:
    f()
```

How can we make it print out what we apparently want?

---
### [Problem 189](solutions/problem_189.js)
#### Difficulty: `Easy`

This problem was asked by Google.

Given an array of elements, return the length of the longest subarray where all its elements are distinct.

For example, given the array `[5, 1, 3, 5, 2, 3, 4, 1]`, return `5` as the longest subarray of distinct elements is `[5, 2, 3, 4, 1]`.

---
### [Problem 190](solutions/problem_190.js)
#### Difficulty: `Easy`

This problem was asked by Facebook.

Given a circular array, compute its maximum subarray sum in `O(n)` time. A subarray can be empty, and in this case the sum is `0`.

For example, given `[8, -1, 3, 4]`, return `15` as we choose the numbers `3`, `4`, and `8` where the `8` is obtained from wrapping around.

Given `[-4, 5, 1, 0]`, return `6` as we choose the numbers `5` and `1`.

---
### [Problem 191](solutions/problem_191.js)
#### Difficulty: `Easy`

This problem was asked by Stripe.

Given a collection of intervals, find the minimum number of intervals you need to remove to make the rest of the intervals non-overlapping.

Intervals can "touch", such as `[0, 1]` and `[1, 2]`, but they won't be considered overlapping.

For example, given the intervals `[7, 9]`, `[2, 4]`, `[5, 8]`, return `1` as the last interval can be removed and the first two won't overlap.

The intervals are not necessarily sorted in any order.

---
### [Problem 192](solutions/problem_192.js)
#### Difficulty: `Medium`

This problem was asked by Google.

You are given an array of non-negative integers. Let's say you start at the beginning of the array and are trying to advance to the end. You can advance at most, the number of steps that you're currently on. Determine whether you can get to the end of the array.

For example, given the array `[1, 3, 1, 2, 0, 1]`, we can go from indices `0 -> 1 -> 3 -> 5`, so return `true`.

Given the array `[1, 2, 1, 0, 0]`, we can't reach the end, so return `false`.

---
### [Problem 193](solutions/problem_193.js)
#### Difficulty: `Hard`

This problem was asked by Affirm.

Given a array of numbers representing the stock prices of a company in chronological order, write a function that calculates the maximum profit you could have made from buying and selling that stock. You're also given a number fee that represents a transaction fee for each buy and sell transaction.

You must buy before you can sell the stock, but you can make as many transactions as you like.

For example, given `[1, 3, 2, 8, 4, 10]` and `fee = 2`, you should return `9`, since you could buy the stock at `1` dollar, and sell at `8` dollars, and then buy it at `4` dollars and sell it at `10` dollars. Since we did two transactions, there is a `4` dollar fee, so we have `7 + 6 = 13` profit minus `4` dollars of fees.

---
### [Problem 194](solutions/problem_194.js)
#### Difficulty: `Easy`

This problem was asked by Facebook.

Suppose you are given two lists of n points, one list p1, p2, ..., pn on the line y = 0 and the other list q1, q2, ..., qn on the line y = 1. Imagine a set of n line segments connecting each point pi to qi. Write an algorithm to determine how many pairs of the line segments intersect.

---
### [Problem 195](solutions/problem_195.js)
#### Difficulty: `Hard`

This problem was asked by Google.

Let A be an `N` by `M` matrix in which every row and every column is sorted.

Given `i1`, `j1`, `i2`, and `j2`, compute the number of elements of `M` smaller than `M[i1, j1]` and larger than `M[i2, j2]`.

For example, given the following matrix:

```
[[1, 3, 7, 10, 15, 20],
 [2, 6, 9, 14, 22, 25],
 [3, 8, 10, 15, 25, 30],
 [10, 11, 12, 23, 30, 35],
 [20, 25, 30, 35, 40, 45]]
```

And `i1 = 1`, `j1 = 1`, `i2 = 3`, `j2 = 3`, return `15` as there are `15` numbers in the matrix smaller than `6` or greater than `23`.

---
### [Problem 196](solutions/problem_196.js)
#### Difficulty: `Easy`

This problem was asked by Apple.

Given the root of a binary tree, find the most frequent subtree sum. The subtree sum of a node is the sum of all values under a node, including the node itself.

For example, given the following tree:

```
  5
 / \
2  -5
```

Return `2` as it occurs twice: once as the left leaf, and once as the sum of `2 + 5 - 5`.

---
### [Problem 197](solutions/problem_197.js)
#### Difficulty: `Easy`

This problem was asked by Amazon.

Given an array and a number `k` that's smaller than the length of the array, rotate the array to the right `k` elements in-place.

---
### [Problem 198](solutions/problem_198.js)
#### Difficulty: `Medium`

This problem was asked by Google.

Given a set of distinct positive integers, find the largest subset such that every pair of elements in the subset `(i, j)` satisfies either `i % j = 0` or `j % i = 0`.

For example, given the set `[3, 5, 10, 20, 21]`, you should return `[5, 10, 20]`. Given `[1, 3, 6, 24]`, return `[1, 3, 6, 24]`.

---
### [Problem 199](solutions/problem_199.js)
#### Difficulty: `Hard`

This problem was asked by Facebook.

Given a string of parentheses, find the balanced string that can be produced from it using the minimum number of insertions and deletions. If there are multiple solutions, return any of them.

For example, given `(()`, you could return `(())`. Given `))()(`, you could return `()()()()`.

---
### [Problem 200](solutions/problem_200.js)
#### Difficulty: `Hard`

This problem was asked by Microsoft.

Let `X` be a set of n intervals on the real line. We say that a set of points `P` "stabs" `X` if every interval in `X` contains at least one point in `P`. Compute the smallest set of points that stabs `X`.

For example, given the intervals `[(1, 4), (4, 5), (7, 9), (9, 12)]`, you should return `[4, 9]`.

---
### [Problem 201](solutions/problem_201.js)
#### Difficulty: `Easy`

This problem was asked by Google.

You are given an array of arrays of integers, where each array corresponds to a row in a triangle of numbers. For example, `[[1], [2, 3], [1, 5, 1]]` represents the triangle:

```
  1
 2 3
1 5 1
```

We define a path in the triangle to start at the top and go down one row at a time to an adjacent value, eventually ending with an entry on the bottom row. For example, `1 -> 3 -> 5`. The weight of the path is the sum of the entries.

Write a program that returns the weight of the maximum weight path.

---
### [Problem 202](solutions/problem_202.js)
#### Difficulty: `Easy`

This problem was asked by Palantir.

Write a program that checks whether an integer is a palindrome. For example, `121` is a palindrome, as well as `888`. `678` is not a palindrome. Do not convert the integer into a string.

---
### [Problem 203](solutions/problem_203.js)
#### Difficulty: `Medium`

This problem was asked by Uber.

Suppose an array sorted in ascending order is rotated at some pivot unknown to you beforehand. Find the minimum element in `O(log N)` time. You may assume the array does not contain duplicates.

For example, given `[5, 7, 10, 3, 4]`, return `3`.

---
### [Problem 204](solutions/problem_204.js)
#### Difficulty: `Easy`

This problem was asked by Amazon.

Given a complete binary tree, count the number of nodes in faster than `O(n)` time. Recall that a complete binary tree has every level filled except the last, and the nodes in the last level are filled starting from the left.

---
### [Problem 205](solutions/problem_205.js)
#### Difficulty: `Easy`

This problem was asked by IBM.

Given an integer, find the next permutation of it in absolute order. For example, given `48975`, the next permutation would be `49578`.

---
### [Problem 206](solutions/problem_206.js)
#### Difficulty: `Easy`

This problem was asked by Twitter.

A permutation can be specified by an array `P`, where `P[i]` represents the location of the element at `i` in the permutation. For example, [`2, 1, 0]` represents the permutation where elements at the index `0` and `2` are swapped.

Given an array and a permutation, apply the permutation to the array. For example, given the array `["a", "b", "c"]` and the permutation `[2, 1, 0]`, return `["c", "b", "a"]`.

---
### [Problem 207](solutions/problem_207.js)
#### Difficulty: `Medium`

This problem was asked by Dropbox.

Given an undirected graph `G`, check whether it is bipartite. Recall that a graph is bipartite if its vertices can be divided into two independent sets, `U` and `V`, such that no edge connects vertices of the same set.

---
### [Problem 208](solutions/problem_208.js)
#### Difficulty: `Medium`

This problem was asked by LinkedIn.

Given a linked list of numbers and a pivot `k`, partition the linked list so that all nodes less than `k` come before nodes greater than or equal to `k`.

For example, given the linked list `5 -> 1 -> 8 -> 0 -> 3` and `k = 3`, the solution could be `1 -> 0 -> 5 -> 8 -> 3`.

---
### [Problem 209](solutions/problem_209.js)
#### Difficulty: `Hard`

This problem was asked by YouTube.

Write a program that computes the length of the longest common subsequence of three given strings. For example, given `epidemiologist`, `refrigeration`, and `supercalifragilisticexpialodocious`, it should return `5`, since the longest common subsequence is `eieio`.

---
### [Problem 210](solutions/problem_210.js)
#### Difficulty: `Easy`

This problem was asked by Apple.

A Collatz sequence in mathematics can be defined as follows. Starting with any positive integer:

- if `n` is even, the next number in the sequence is `n / 2`
- if `n` is odd, the next number in the sequence is `3n + 1`

It is conjectured that every such sequence eventually reaches the number `1`. Test this conjecture.

Bonus: What input `n <= 1000000` gives the longest sequence?

---
### [Problem 211](solutions/problem_211.js)
#### Difficulty: `Easy`

This problem was asked by Microsoft.

Given a string and a pattern, find the starting indices of all occurrences of the pattern in the string. For example, given the string `abracadabra` and the pattern `abr`, you should return `[0, 7]`.

---
### [Problem 212](solutions/problem_212.js)
#### Difficulty: `Easy`

This problem was asked by Dropbox.

Spreadsheets often use this alphabetical encoding for its columns: "A", "B", "C", ..., "AA", "AB", ..., "ZZ", "AAA", "AAB", ....

Given a column number, return its alphabetical column id. For example, given `1`, return "A". Given `27`, return "AA".

---
### [Problem 213](solutions/problem_213.js)
#### Difficulty: `Medium`

This problem was asked by Snapchat.

Given a string of digits, generate all possible valid IP address combinations.

IP addresses must follow the format `A.B.C.D`, where `A`, `B`, `C`, and `D` are numbers between `0` and `255`. Zero-prefixed numbers, such as `01` and `065`, are not allowed, except for `0` itself.

For example, given `2542540123`, you should return `['254.25.40.123', '254.254.0.123']`.

---
### [Problem 214](solutions/problem_214.js)
#### Difficulty: `Easy`

This problem was asked by Stripe.

Given an integer `n`, return the length of the longest consecutive run of `1`s in its binary representation.

For example, given `156`, you should return `3`.

---
### [Problem 215](solutions/problem_215.js)
#### Difficulty: `Medium`

This problem was asked by Yelp.

The horizontal distance of a binary tree node describes how far left or right the node will be when the tree is printed out.

More rigorously, we can define it as follows:

- The horizontal distance of the root is `0`.
- The horizontal distance of a left child is `hd(parent) - 1`.
- The horizontal distance of a right child is `hd(parent) + 1`.

For example, for the following tree, `hd(1) = -2`, and `hd(6) = 0`.

```
             5
          /     \
        3         7
      /  \      /   \
    1     4    6     9
   /                /
  0                8
```

The bottom view of a tree, then, consists of the lowest node at each horizontal distance. If there are two nodes at the same depth and horizontal distance, either is acceptable.

For this tree, for example, the bottom view could be `[0, 1, 3, 6, 8, 9]`.

Given the root to a binary tree, return its bottom view.

---
### [Problem 216](solutions/problem_216.js)
#### Difficulty: `Medium`

This problem was asked by Facebook.

Given a number in Roman numeral format, convert it to decimal.

The values of Roman numerals are as follows:

```
{
    'M': 1000,
    'D': 500,
    'C': 100,
    'L': 50,
    'X': 10,
    'V': 5,
    'I': 1
}
```

In addition, note that the Roman numeral system uses subtractive notation for numbers such as `IV` and `XL`.

For the input `XIV`, for instance, you should return `14`.

---
### [Problem 217](solutions/problem_217.js)
#### Difficulty: `Hard`

This problem was asked by Oracle.

We say a number is sparse if there are no adjacent ones in its binary representation. For example, `21` (`10101`) is sparse, but `22` (`10110`) is not. For a given input `N`, find the smallest sparse number greater than or equal to `N`.

Do this in faster than `O(N log N)` time.

---
### [Problem 218](solutions/problem_218.js)
#### Difficulty: `Medium`

This problem was asked by Yahoo.

Write an algorithm that computes the reversal of a directed graph. For example, if a graph consists of `A -> B -> C`, it should become `A <- B <- C`.

---
### [Problem 219](solutions/problem_219.js)
#### Difficulty: `Hard`

This problem was asked by Salesforce.

Connect 4 is a game where opponents take turns dropping red or black discs into a `7 x 6` vertically suspended grid. The game ends either when one player creates a line of four consecutive discs of their color (horizontally, vertically, or diagonally), or when there are no more spots left in the grid.

Design and implement Connect 4.

---
### [Problem 220](solutions/problem_220.js)
#### Difficulty: `Medium`

This problem was asked by Square.

In front of you is a row of N coins, with values `v1`, `v1`, `...,` `vn`.

You are asked to play the following game. You and an opponent take turns choosing either the first or last coin from the row, removing it from the row, and receiving the value of the coin.

Write a program that returns the maximum amount of money you can win with certainty, if you move first, assuming your opponent plays optimally.

---
### [Problem 221](solutions/problem_221.js)
#### Difficulty: `Easy`

This problem was asked by Zillow.

Let's define a "sevenish" number to be one which is either a power of `7`, or the sum of unique powers of `7`. The first few sevenish numbers are `1`, `7`, `8`, `49`, and so on. Create an algorithm to find the `nth` sevenish number.

---
### [Problem 222](solutions/problem_222.js)
#### Difficulty: `Medium`

This problem was asked by Quora.

Given an absolute pathname that may have `.` or `..` as part of it, return the shortest standardized path.

For example, given `/usr/bin/../bin/./scripts/../`, return `/usr/bin/`.

---
### [Problem 223](solutions/problem_223.js)
#### Difficulty: `Hard`

This problem was asked by Palantir.

Typically, an implementation of in-order traversal of a binary tree has `O(h)` space complexity, where `h` is the height of the tree. Write a program to compute the in-order traversal of a binary tree using `O(1)` space.

---
### [Problem 224](solutions/problem_224.js)
#### Difficulty: `Easy`

This problem was asked by Amazon.

Given a sorted array, find the smallest positive integer that is not the sum of a subset of the array.

For example, for the input `[1, 2, 3, 10]`, you should return `7`.

Do this in `O(N)` time.

---
### [Problem 225](solutions/problem_225.js)
#### Difficulty: `Easy`

This problem was asked by Bloomberg.

There are `N` prisoners standing in a circle, waiting to be executed. The executions are carried out starting with the `kth` person, and removing every successive `kth` person going clockwise until there is no one left.

Given `N` and `k`, write an algorithm to determine where a prisoner should stand in order to be the last survivor.

For example, if `N = 5` and `k = 2`, the order of executions would be `[2, 4, 1, 5, 3]`, so you should return `3`.

Bonus: Find an `O(log N)` solution if `k = 2`.

---
### [Problem 226](solutions/problem_226.js)
#### Difficulty: `Hard`

This problem was asked by Airbnb.

You come across a dictionary of sorted words in a language you've never seen before. Write a program that returns the correct order of letters in this language.

For example, given `['xww', 'wxyz', 'wxyw', 'ywx', 'ywz']`, you should return `['x', 'z', 'w', 'y']`.

---
### [Problem 227](solutions/problem_227.js)
#### Difficulty: `Easy`

This problem was asked by Facebook.

Boggle is a game played on a `4 x 4` grid of letters. The goal is to find as many words as possible that can be formed by a sequence of adjacent letters in the grid, using each cell at most once. Given a game board and a dictionary of valid words, implement a Boggle solver.

---
### [Problem 228](solutions/problem_228.js)
#### Difficulty: `Medium`

This problem was asked by Twitter.

Given a list of numbers, create an algorithm that arranges them in order to form the largest possible integer. For example, given `[10, 7, 76, 415]`, you should return `77641510`.

---
### [Problem 229](solutions/problem_229.js)
#### Difficulty: `Medium`

This problem was asked by Flipkart.

Snakes and Ladders is a game played on a `10` x `10` board, the goal of which is get from square `1` to square `100`. On each turn players will roll a six-sided die and move forward a number of spaces equal to the result. If they land on a square that represents a snake or ladder, they will be transported ahead or behind, respectively, to a new square.

Find the smallest number of turns it takes to play snakes and ladders.

For convenience, here are the squares representing snakes and ladders, and their outcomes:

```
snakes = {16: 6, 48: 26, 49: 11, 56: 53, 62: 19, 64: 60, 87: 24, 93: 73, 95: 75, 98: 78}

ladders = {1: 38, 4: 14, 9: 31, 21: 42, 28: 84, 36: 44, 51: 67, 71: 91, 80: 100}
```

---
### [Problem 230](solutions/problem_230.js)
#### Difficulty: `Medium`

This problem was asked by Goldman Sachs.

You are given `N` identical eggs and access to a building with `k` floors. Your task is to find the lowest floor that will cause an egg to break, if dropped from that floor. Once an egg breaks, it cannot be dropped again. If an egg breaks when dropped from the `xth` floor, you can assume it will also break when dropped from any floor greater than `x`.

Write an algorithm that finds the minimum number of trial drops it will take, in the worst case, to identify this floor.

For example, if `N = 1` and `k = 5`, we will need to try dropping the egg at every floor, beginning with the first, until we reach the fifth floor, so our solution will be `5`.

---
### [Problem 231](solutions/problem_231.js)
#### Difficulty: `Easy`

This problem was asked by IBM.

Given a string with repeated characters, rearrange the string so that no two adjacent characters are the same. If this is not possible, return `None`.

For example, given `aaabbc`, you could return `ababac`. Given `aaab`, return `None`.

---
### [Problem 232](solutions/problem_232.js)
#### Difficulty: `Easy`

This problem was asked by Google.

Implement a `PrefixMapSum` class with the following methods:

- `insert(key: str, value: int)`: Set a given key's value in the map. If the key already exists, overwrite the value.
- `sum(prefix: str)`: Return the sum of all values of keys that begin with a given prefix.

For example, you should be able to run the following code:

```
mapsum.insert("columnar", 3)
assert mapsum.sum("col") == 3
```

```
mapsum.insert("column", 2)
assert mapsum.sum("col") == 5
```

---
### [Problem 233](solutions/problem_233.js)
#### Difficulty: `Easy`

This problem was asked by Apple.

Implement the function `fib(n)`, which returns the `nth` number in the Fibonacci sequence, using only `O(1)` space.

---
### [Problem 234](solutions/problem_234.js)
#### Difficulty: `Hard`

This problem was asked by Microsoft.

Recall that the minimum spanning tree is the subset of edges of a tree that connect all its vertices with the smallest possible total edge weight. Given an undirected graph with weighted edges, compute the _maximum_ weight spanning tree.

---
### [Problem 235](solutions/problem_235.js)
#### Difficulty: `Hard`

This problem was asked by Facebook.

Given an array of numbers of length `N`, find both the minimum and maximum using less than `2 * (N - 2)` comparisons.

---
### [Problem 236](solutions/problem_236.js)
#### Difficulty: `Medium`

This problem was asked by Nvidia.

You are given a list of `N` points `(x1, y1)`, `(x2, y2)`, `...`, `(xN, yN)` representing a polygon. You can assume these points are given in order; that is, you can construct the polygon by connecting point `1` to point `2`, point `2` to point `3`, and so on, finally looping around to connect point `N` to point `1`.

Determine if a new point `p` lies inside this polygon. (If `p` is on the boundary of the polygon, you should return `False`).

---
### [Problem 237](solutions/problem_237.js)
#### Difficulty: `Easy`

This problem was asked by Amazon.

A tree is symmetric if its data and shape remain unchanged when it is reflected about the root node. The following tree is an example:

```
        4
      / | \
    3   5   3
  /           \
9              9
```

Given a `k`-ary tree, determine whether it is symmetric.

---
### [Problem 238](solutions/problem_238.js)
#### Difficulty: `Hard`

This problem was asked by MIT.

[Blackjack](https://en.wikipedia.org/wiki/Blackjack) is a two player card game whose rules are as follows:

- The player and then the dealer are each given two cards.
- The player can then "hit", or ask for arbitrarily many additional cards, so long as their total does not exceed `21`.
- The dealer must then hit if their total is `16` or lower, otherwise pass.
- Finally, the two compare totals, and the one with the greatest sum not exceeding `21` is the winner.

For this problem, cards values are counted as follows: each card between `2` and `10` counts as their face value, face cards count as `10`, and aces count as `1`.

Given perfect knowledge of the sequence of cards in the deck, implement a blackjack solver that maximizes the player's score (that is, wins minus losses).

---
### [Problem 239](solutions/problem_239.js)
#### Difficulty: `Medium`

This problem was asked by Uber.

One way to unlock an Android phone is through a pattern of swipes across a `1-9` keypad.

For a pattern to be valid, it must satisfy the following:

- All of its keys must be distinct.
- It must not connect two keys by jumping over a third key, unless that key has already been used.

For example, `4 - 2 - 1 - 7` is a valid pattern, whereas `2 - 1 - 7` is not.

Find the total number of valid unlock patterns of length `N`, where `1 <= N <= 9`.

---
### [Problem 240](solutions/problem_240.js)
#### Difficulty: `Hard`

This problem was asked by Spotify.

There are `N` couples sitting in a row of length `2 * N`. They are currently ordered randomly, but would like to rearrange themselves so that each couple's partners can sit side by side.

What is the minimum number of swaps necessary for this to happen?

---
### [Problem 241](solutions/problem_241.js)
#### Difficulty: `Easy`

This problem was asked by Palantir.

In academia, the `h`-index is a metric used to calculate the impact of a researcher's papers. It is calculated as follows:

A researcher has index `h` if at least `h` of her `N` papers have `h` citations each. If there are multiple `h` satisfying this formula, the maximum is chosen.

For example, suppose` N = 5`, and the respective citations of each paper are `[4, 3, 0, 1, 5]`. Then the `h`-index would be `3`, since the researcher has `3` papers with at least `3` citations.

Given a list of paper citations of a researcher, calculate their h-index.

---
### [Problem 242](solutions/problem_242.js)
#### Difficulty: `Hard`

This problem was asked by Twitter.

You are given an array of length `24`, where each element represents the number of new subscribers during the corresponding hour. Implement a data structure that efficiently supports the following:

- `update(hour: int, value: int)`: Increment the element at index hour by value.
- `query(start: int, end: int)`: Retrieve the number of subscribers that have signed up between start and end (inclusive).
You can assume that all values get cleared at the end of the day, and that you will not be asked for start and end values that wrap around midnight.

---
### [Problem 243](solutions/problem_243.js)
#### Difficulty: `Hard`

This problem was asked by Etsy.

Given an array of numbers `N` and an integer `k`, your task is to split `N` into `k` partitions such that the maximum sum of any partition is minimized. Return this `sum`.

For example, given `N = [5, 1, 2, 7, 3, 4]` and `k = 3`, you should return `8`, since the optimal partition is `[5, 1, 2]`, `[7]`, `[3, 4]`.

---
### [Problem 244](solutions/problem_244.js)
#### Difficulty: `Easy`

This problem was asked by Square.

The Sieve of Eratosthenes is an algorithm used to generate all prime numbers smaller than `N`. The method is to take increasingly larger prime numbers, and mark their multiples as composite.

For example, to find all primes less than `100`, we would first mark `[4, 6, 8, ...]` (multiples of two), then `[6, 9, 12, ...]` (multiples of three), and so on. Once we have done this for all primes less than `N`, the unmarked numbers that remain will be prime.

Implement this algorithm.

Bonus: Create a generator that produces primes indefinitely (that is, without taking `N` as an input).

---
### [Problem 245](solutions/problem_245.js)
#### Difficulty: `Medium`

This problem was asked by Yelp.

You are given an array of integers, where each element represents the maximum number of steps that can be jumped going forward from that element. Write a function to return the minimum number of jumps you must take in order to get from the start to the end of the array.

For example, given `[6, 2, 4, 0, 5, 1, 1, 4, 2, 9]`, you should return `2`, as the optimal solution involves jumping from `6` to `5`, and then from `5` to `9`.

---
### [Problem 246](solutions/problem_246.js)
#### Difficulty: `Medium`

This problem was asked by Dropbox.

Given a list of words, determine whether the words can be chained to form a circle. A word `X` can be placed in front of another word `Y` in a circle if the last character of `X` is same as the first character of `Y`.

For example, the words `['chair', 'height', 'racket', touch', 'tunic']` can form the following circle: `chair --> racket --> touch --> height --> tunic --> chair`.

---
### [Problem 247](solutions/problem_247.js)
#### Difficulty: `Easy`

This problem was asked by PayPal.

Given a binary tree, determine whether or not it is height-balanced. A height-balanced binary tree can be defined as one in which the heights of the two subtrees of any node never differ by more than one.

---
### [Problem 248](solutions/problem_248.js)
#### Difficulty: `Hard`

This problem was asked by Nvidia.

Find the maximum of two numbers without using any if-else statements, branching, or direct comparisons.

---
### [Problem 249](solutions/problem_249.js)
#### Difficulty: `Hard`

This problem was asked by Salesforce.

Given an array of integers, find the maximum XOR of any two elements.

---
### [Problem 250](solutions/problem_250.js)
#### Difficulty: `Medium`

This problem was asked by Google.

A cryptarithmetic puzzle is a mathematical game where the digits of some numbers are represented by letters. Each letter represents a unique digit.

For example, a puzzle of the form:

```
  SEND
+ MORE
--------
 MONEY
```

may have the solution:

`{'S': 9, 'E': 5, 'N': 6, 'D': 7, 'M': 1, 'O', 0, 'R': 8, 'Y': 2}`

Given a three-word puzzle like the one above, create an algorithm that finds a solution.

---
### [Problem 251](solutions/problem_251.js)
#### Difficulty: `Medium`

This problem was asked by Amazon.

Given an array of a million integers between zero and a billion, out of order, how can you efficiently sort it? Assume that you cannot store an array of a billion elements in memory.

---
### [Problem 252](solutions/problem_252.js)
#### Difficulty: `Medium`

This problem was asked by Palantir.

The ancient Egyptians used to express fractions as a sum of several terms where each numerator is one. For example, `4 / 13` can be represented as `1 / 4 + 1 / 18 + 1 / 468`.

Create an algorithm to turn an ordinary fraction `a / b`, where `a < b`, into an Egyptian fraction.

---
### [Problem 253](solutions/problem_253.js)
#### Difficulty: `Medium`

This problem was asked by PayPal.

Given a string and a number of lines `k`, print the string in zigzag form. In zigzag, characters are printed out diagonally from top left to bottom right until reaching the kth line, then back up to top right, and so on.

For example, given the sentence `thisisazigzag` and `k = 4`, you should print:

```
t     a     g
 h   s z   a
  i i   i z
   s     g
```

---
### [Problem 254](solutions/problem_254.js)
#### Difficulty: `Medium`

This problem was asked by Yahoo.

Recall that a full binary tree is one in which each node is either a leaf node, or has two children. Given a binary tree, convert it to a full one by removing nodes with only one child.

For example, given the following tree:

```
         0
      /     \
    1         2
  /            \
3                 4
  \             /   \
    5          6     7
```

You should convert it to:

```
     0
  /     \
5         4
        /   \
       6     7
```

---
### [Problem 255](solutions/problem_255.js)
#### Difficulty: `Easy`

This problem was asked by Microsoft.

The transitive closure of a graph is a measure of which vertices are reachable from other vertices. It can be represented as a matrix `M`, where `M[i][j] == 1` if there is a path between vertices `i` and `j`, and otherwise `0`.

For example, suppose we are given the following graph in adjacency list form:

```
graph = [
    [0, 1, 3],
    [1, 2],
    [2],
    [3]
]
```

The transitive closure of this graph would be:

```
[1, 1, 1, 1]
[0, 1, 1, 0]
[0, 0, 1, 0]
[0, 0, 0, 1]
```

Given a graph, find its transitive closure.

---
### [Problem 256](solutions/problem_256.js)
#### Difficulty: `Medium`

This problem was asked by Fitbit.

Given a linked list, rearrange the node values such that they appear in alternating `low -> high -> low -> high ...` form. For example, given `1 -> 2 -> 3 -> 4 -> 5`, you should return `1 -> 3 -> 2 -> 5 -> 4`.

---
### [Problem 257](solutions/problem_257.js)
#### Difficulty: `Medium`

This problem was asked by WhatsApp.

Given an array of integers out of order, determine the bounds of the smallest window that must be sorted in order for the entire array to be sorted. For example, given `[3, 7, 5, 6, 9]`, you should return `(1, 3)`.

---
### [Problem 258](solutions/problem_258.js)
#### Difficulty: `Easy`

This problem was asked by Morgan Stanley.

In Ancient Greece, it was common to write text with the first line going left to right, the second line going right to left, and continuing to go back and forth. This style was called "`boustrophedon`".

Given a binary tree, write an algorithm to print the nodes in boustrophedon order.

For example, given the following tree:

```
       1
    /     \
  2         3
 / \       / \
4   5     6   7
```
You should return `[1, 3, 2, 4, 5, 6, 7]`.

---
### [Problem 259](solutions/problem_259.js)
#### Difficulty: `Hard`

This problem was asked by Two Sigma.

Ghost is a two-person word game where players alternate appending letters to a word. The first person who spells out a word, or creates a prefix for which there is no possible continuation, loses. Here is a sample game:

Player 1: `g`
Player 2: `h`
Player 1: `o`
Player 2: `s`
Player 1: `t` [loses]

Given a dictionary of words, determine the letters the first player should start with, such that with optimal play they cannot lose.

For example, if the dictionary is `["cat", "calf", "dog", "bear"]`, the only winning start letter would be `b`.

---
### [Problem 260](solutions/problem_260.js)
#### Difficulty: `Medium`

This problem was asked by Pinterest.

The sequence `[0, 1, ..., N]` has been jumbled, and the only clue you have for its order is an array representing whether each number is larger or smaller than the last. Given this information, reconstruct an array that is consistent with it. For example, given `[null, +, +, -, +]`, you could return `[1, 2, 3, 0, 4]`.

---
### [Problem 261](solutions/problem_261.js)
#### Difficulty: `Easy`

This problem was asked by Amazon.

Huffman coding is a method of encoding characters based on their frequency. Each letter is assigned a variable-length binary string, such as `0101` or `111110`, where shorter lengths correspond to more common letters. To accomplish this, a binary tree is built such that the path from the root to any leaf uniquely maps to a character. When traversing the path, descending to a left child corresponds to a `0` in the prefix, while descending right corresponds to `1`.

Here is an example tree (note that only the leaf nodes have letters):

```
        *
      /   \
    *       *
   / \     / \
  *   a   t   *
 /             \
c               s
```

With this encoding, cats would be represented as `0000110111`.

Given a dictionary of character frequencies, build a Huffman tree, and use it to determine a mapping between characters and their encoded binary strings.

---
### [Problem 262](solutions/problem_262.js)
#### Difficulty: `Medium`

This problem was asked by Mozilla.

A bridge in a connected (undirected) graph is an edge that, if removed, causes the graph to become disconnected. Find all the bridges in a graph.

---
### [Problem 263](solutions/problem_263.js)
#### Difficulty: `Medium`

This problem was asked by Nest.

Create a basic sentence checker that takes in a stream of characters and determines whether they form valid sentences. If a sentence is valid, the program should print it out.

We can consider a sentence valid if it conforms to the following rules:

1. The sentence must start with a capital letter, followed by a lowercase letter or a space.
1. All other characters must be lowercase letters, separators (`,`,`;`,`:`) or terminal marks (`.`,`?`,`!`,`‽`).
1. There must be a single space between each word.
1. The sentence must end with a terminal mark immediately following a word.

---
### [Problem 264](solutions/problem_264.js)
#### Difficulty: `Hard`

This problem was asked by LinkedIn.

Given a set of characters `C` and an integer `k`, a De Bruijn sequence is a cyclic sequence in which every possible k-length string of characters in C occurs exactly once.

For example, suppose `C = {0, 1}` and `k = 3`. Then our sequence should contain the substrings `{'000', '001', '010', '011', '100', '101', '110', '111'}`, and one possible solution would be `00010111`.

Create an algorithm that finds a De Bruijn sequence.

---
### [Problem 265](solutions/problem_265.js)
#### Difficulty: `Easy`

This problem was asked by Atlassian.

MegaCorp wants to give bonuses to its employees based on how many lines of codes they have written. They would like to give the smallest positive amount to each worker consistent with the constraint that if a developer has written more lines of code than their neighbor, they should receive more money.

Given an array representing a line of seats of employees at MegaCorp, determine how much each one should get paid.

For example, given `[10, 40, 200, 1000, 60, 30]`, you should return `[1, 2, 3, 4, 2, 1]`.

---
### [Problem 266](solutions/problem_266.js)
#### Difficulty: `Easy`

This problem was asked by Pivotal.

A step word is formed by taking a given word, adding a letter, and anagramming the result. For example, starting with the word `APPLE`, you can add an `A` and anagram to get `APPEAL`.

Given a dictionary of words and an input word, create a function that returns all valid step words.

---
### [Problem 267](solutions/problem_267.js)
#### Difficulty: `Hard`

This problem was asked by Oracle.

You are presented with an `8` by `8` matrix representing the positions of pieces on a chess board. The only pieces on the board are the black king and various white pieces. Given this matrix, determine whether the king is in check.

For details on how each piece moves, see here.

For example, given the following matrix:

```
...K....
........
.B......
......P.
.......R
..N.....
........
.....Q..
```

You should return `True`, since the bishop is attacking the king diagonally.

---
### [Problem 268](solutions/problem_268.js)
#### Difficulty: `Medium`

This problem was asked by Indeed.

Given a `32`-bit positive integer `N`, determine whether it is a power of four in faster than `O(log N)` time.

---
### [Problem 269](solutions/problem_269.js)
#### Difficulty: `Easy`

This problem was asked by Microsoft.

You are given an string representing the initial conditions of some dominoes. Each element can take one of three values:

- `L`, meaning the domino has just been pushed to the left,
- `R`, meaning the domino has just been pushed to the right, or
- `.`, meaning the domino is standing still.

Determine the orientation of each tile when the dominoes stop falling. Note that if a domino receives a force from the left and right side simultaneously, it will remain upright.

For example, given the string `.L.R....L`, you should return `LL.RRRLLL`.

Given the string `..R...L.L`, you should return `..RR.LLLL`.

---
### [Problem 270](solutions/problem_270.js)
#### Difficulty: `Medium`

This problem was asked by Twitter.

A network consists of nodes labeled `0` to `N`. You are given a list of edges `(a, b, t)`, describing the time `t` it takes for a message to be sent from node `a` to node `b`. Whenever a node receives a message, it immediately passes the message on to a neighboring node, if possible.

Assuming all nodes are connected, determine how long it will take for every node to receive a message that begins at node `0`.

For example, given `N = 5`, and the following edges:

```
edges = [
    (0, 1, 5),
    (0, 2, 3),
    (0, 5, 4),
    (1, 3, 8),
    (2, 3, 1),
    (3, 5, 10),
    (3, 4, 5)
]
```

You should return `9`, because propagating the message from `0 -> 2 -> 3 -> 4` will take that much time.

---
### [Problem 271](solutions/problem_271.js)
#### Difficulty: `Hard`

This problem was asked by Netflix.

Given a sorted list of integers of length `N`, determine if an element `x` is in the list without performing any multiplication, division, or bit-shift operations.

Do this in `O(log N)` time.

---
### [Problem 272](solutions/problem_272.js)
#### Difficulty: `Medium`

This problem was asked by Spotify.

Write a function, `throw_dice(N, faces, total)`, that determines how many ways it is possible to throw `N` dice with some number of faces each to get a specific total.

For example, `throw_dice(3, 6, 7)` should equal `15`.

---
### [Problem 273](solutions/problem_273.js)
#### Difficulty: `Easy`

This problem was asked by Apple.

A fixed point in an array is an element whose value is equal to its index. Given a sorted array of distinct elements, return a fixed point, if one exists. Otherwise, return `False`.

For example, given `[-6, 0, 2, 40]`, you should return `2`. Given `[1, 5, 7, 8]`, you should return `False`.

---
### [Problem 274](solutions/problem_274.js)
#### Difficulty: `Hard`

Given a string consisting of parentheses, single digits, and positive and negative signs, convert the string into a mathematical expression to obtain the answer.

Don't use `eval` or a similar built-in parser.

For example, given `'-1 + (2 + 3)'`, you should return `4`.

---
### [Problem 275](solutions/problem_275.js)
#### Difficulty: `Medium`

This problem was asked by Epic.

The "look and say" sequence is defined as follows: beginning with the term `1`, each subsequent term visually describes the digits appearing in the previous term. The first few terms are as follows:

```
1
11
21
1211
111221
```

As an example, the fourth term is `1211`, since the third term consists of one `2` and one `1`.

Given an integer `N`, print the `N`th term of this sequence.

---
### [Problem 276](solutions/problem_276.js)
#### Difficulty: `Hard`

This problem was asked by Dropbox.

Implement an efficient string matching algorithm.

That is, given a string of length `N` and a pattern of length `k`, write a program that searches for the pattern in the string with less than `O(N * k)` worst-case time complexity.

If the pattern is found, return the start index of its location. If not, return `False`.

---