Completed PreCourse-1

Exercise_1.py

@@ -1,21 +1,31 @@
+#Time Complexity: o(1)
+# #Space Complexity:0(n)
 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 len(self.stack)==0
+
      def push(self, item):
+          self.stack.append(item)
 
      def pop(self):
-
+        if self.isEmpty():
+             return None
+        return self.stack.pop()
 
      def peek(self):
-
+        if self.isEmpty():
+             return None
+        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

@@ -1,15 +1,24 @@
-
+#Time Complexity: O(1)
+#Space Complexity: O(n)
 class Node:
     def __init__(self, data):
        self.data = data
        self.next = None
 
 class Stack:
     def __init__(self):
+        self.top = None
 
     def push(self, data):
-
-    def pop(self):
+        new_node=Node(data)
+        new_node.next=self.top
+        self.top=new_node    
+def pop(self):
+    if self.top is None:
+        return None
+    popitem = self.top.data
+    self.top=self.top.next
+    return popitem
 
 a_stack = Stack()
 while True:

Exercise_3.py

@@ -1,9 +1,14 @@
+# Time Complexity  : O(n)
+# Space Complexity : O(n) 
+
 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 +22,42 @@ def append(self, data):
         Insert a new element at the end of the list.
         Takes O(n) time.
         """
+        new_node = ListNode(data)           
+        if self.head is None:               
+            self.head = new_node           
+            return
+        current = self.head                 
+        while current.next is not None:     
+            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 is not None:          
+            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.
         """
+        if self.head is None:               
+            return
+        if self.head.data == key:           
+            self.head = self.head.next     
+            return
+        current = self.head                 
+        while current.next is not None:     
+            if current.next.data == key:    
+                current.next = current.next.next  
+                return
+            current = current.next          
+