PreCourse 1 complete

Exercise_1.py

@@ -2,20 +2,25 @@ class myStack:
   #Please read sample.java file before starting.
   #Kindly include Time and Space complexity at top of each file
      def __init__(self):
+          self.stack = []
 
      def isEmpty(self):
+          return self.stack == []
 
      def push(self, item):
+          self.stack.append(item)
 
      def pop(self):
-
+          return self.stack.pop()
 
      def peek(self):
-
+          return self.stack[-1]
+
      def size(self):
-
+          return len(self.stack)
+
      def show(self):
-
+          return self.stack
 
 s = myStack()
 s.push('1')

Exercise_2.py

@@ -6,10 +6,19 @@ def __init__(self, data):
 
 class Stack:
     def __init__(self):
+        self.stack = Node(None)
 
     def push(self, data):
+        new_node = Node(data)
+        new_node.next = self.stack
+        self.stack = new_node
 
     def pop(self):
+        if self.stack.data is None:
+            return None
+        popped_data = self.stack.data
+        self.stack = self.stack.next
+        return popped_data
 
 a_stack = Stack()
 while True:

Exercise_3.py

@@ -3,30 +3,58 @@ 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):
         """
         Create a new singly-linked list.
         Takes O(1) time.
         """
+        self.node = ListNode()
         self.head = None
 
     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
+        else:
+            current = self.head
+            while current.next:
+                current = current.next
+            current.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.
         """
+        current = self.head
+        while current:
+            if current.data == key:
+                return current
+            current = current.next
+        return None
 
     def remove(self, key):
         """
         Remove the first occurrence of `key` in the list.
         Takes O(n) time.
         """
+        current = self.head
+        previous = None
+        while current:
+            if current.data == key:
+                if previous:
+                    previous.next = current.next
+                else:
+                    self.head = current.next
+                return
+            previous = current
+            current = current.next