Design-2 Solution

[indrasena483]

No description provided.

[super30admin]

Create Queue using Stacks (QueueWithStacks.java)

Your solution is excellent! You have correctly implemented the queue using two stacks with amortized constant time operations. Here are some strengths and minor suggestions:

Strengths:

• You have clearly documented the time and space complexity, which shows good understanding.
• The code is clean and follows standard Java practices.
• The use of two stacks and the transfer logic in peek is efficient.

Areas for Improvement:

• In the pop method, you call peek() which doesn't return a value (but you don't use the return value). This is acceptable because you are only interested in the side effect (filling outStack if needed). However, for clarity, you might consider storing the value from peek if you need to return it, but actually you need to pop the element. Alternatively, you could write:

  peek(); // Ensure outStack has the front element
  return outStack.pop();

  But note: the peek method returns the front element without popping. So your current approach is correct: you call peek to ensure outStack is populated, then you pop from outStack. This is efficient and avoids code duplication.

• Since the problem states that all calls to pop and peek are valid, you might not need to check for empty in pop (but it's good to be safe). However, the problem constraints say that there are at most 100 calls, so performance is not critical. But in a real-world scenario, handling edge cases is good.

• The comment in the code is helpful, but you could also add a brief comment in each method to explain what it does (though the code is self-explanatory).

Overall, this is a high-quality solution.

VERDICT: PASS

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Implement Hash Map (HashMap.java)

Strengths:

• The code is very simple and easy to understand.
• It achieves O(1) time complexity for all operations, which is optimal.

Areas for improvement:

• The main issue is the space efficiency. The array always uses 1,000,001 * 4 bytes (for integers) which is about 4MB, regardless of how many keys are stored. This is acceptable for the problem constraints (since the maximum key is 10^6) and given that the problem states at most 10^4 operations, but in general, it is not a flexible design. For example, if the key range were larger (e.g., up to 10^9), this approach would not be feasible.
• The solution does not handle collisions, but in this specific problem, since keys are unique and within the array bounds, collisions (in the sense of different keys mapping to the same index) do not occur. However, the approach is not general and would fail if keys could be negative or beyond 10^6. The problem states that keys are in [0, 10^6], so it is acceptable.
• The solution initializes the array with size 1000001, which is correct because keys can be from 0 to 1000000 (inclusive). However, note that the problem says "0 <= key, value <= 10^6", so the maximum key is 1000000, which requires an array of size 1000001 (index 0 to 1000000).

Alternative approach:

• For a more memory-efficient solution, you could use a dynamic data structure like the reference solution (chaining with linked lists) or open addressing. This would use space proportional to the number of actual key-value pairs stored, which is at most 10^4 (as per the constraints). This would be more efficient in terms of memory when the number of pairs is small.

Overall, the solution is correct and efficient for the given constraints, but it is not flexible and uses more memory than necessary in cases where only a few keys are stored.

VERDICT: PASS