Completed precourse1

Exercise_1.py

@@ -1,20 +1,35 @@
 class myStack:
   #Please read sample.java file before starting.
   #Kindly include Time and Space complexity at top of each file
+  #// Time Complexity : O(1)
+  #// Space Complexity : O(n)
+  #// Did this code successfully run on Leetcode : Yes
+  #// Any problem you faced while coding this : No
+
      def __init__(self):
+        self.items = []
 
      def isEmpty(self):
+        return len(self.items) == 0
 
      def push(self, item):
+        self.items.append(item)
 
      def pop(self):
-
+        if not self.isEmpty():
+            return self.items.pop()
+        else:
+            return "List is empty"
 
      def peek(self):
+        if not self.isEmpty():
+            return self.items[-1]
 
      def size(self):
+        return len(self.items)
 
      def show(self):
+        return self.items
 
 
 s = myStack()

Exercise_2.py

@@ -1,15 +1,28 @@
-
+#// Time Complexity : O(1)
+#// Space Complexity : O(n)
+#// Did this code successfully run on Leetcode : Yes
+#// Any problem you faced while coding this : No
+
 class Node:
     def __init__(self, data):
        self.data = data
        self.next = None
 
 class Stack:
     def __init__(self):
+        self.root = None
 
     def push(self, data):
+        new_node = Node(data)
+        new_node.next = self.root
+        self.root = new_node
 
     def pop(self):
+        if self.root == None:
+            return None
+        node_popped = self.root
+        self.root = self.root.next
+        return node_popped.data
 
 a_stack = Stack()
 while True:
@@ -29,4 +42,4 @@ def pop(self):
         else:
             print('Popped value: ', int(popped))
     elif operation == 'quit':
-        break
+        break

Exercise_3.py

@@ -1,8 +1,14 @@
+#// Time Complexity : O(n)
+#// Space Complexity : O(1)
+#// Did this code successfully run on Leetcode : Yes
+#// Any problem you faced while coding this : No
 class ListNode:
     """
     A node in a singly-linked list.
     """
     def __init__(self, data=None, next=None):
+        self.data = data
+        self.next = next
 
 class SinglyLinkedList:
     def __init__(self):
@@ -17,16 +23,48 @@ def append(self, data):
         Insert a new element at the end of the list.
         Takes O(n) time.
         """
+        new_node = ListNode(data)
+        if not self.head:
+            self.head = new_node
+            return
+
+        # Traverse to the end
+        curr = self.head
+        while curr.next:
+            curr = curr.next
+        curr.next = new_node
+
 
     def find(self, key):
         """
         Search for the first element with `data` matching
         `key`. Return the element or `None` if not found.
         Takes O(n) time.
         """
+        curr = self.head
+        while curr:
+            if curr.data == key:
+                return curr
+            curr = curr.next
+        return None
 
     def remove(self, key):
         """
         Remove the first occurrence of `key` in the list.
         Takes O(n) time.
         """
+        curr = self.head
+        prev = None
+
+        while curr:
+            if curr.data == key:
+                if prev:
+                    #skip current node
+                    prev.next = curr.next
+                else:
+                    #remove the head
+                    self.head = curr.next
+                return True
+            prev = curr
+            curr = curr.next
+        return False