Completed PRECOURSE-1

Exercise_1.py

@@ -1,20 +1,29 @@
 class myStack:
-  #Please read sample.java file before starting.
-  #Kindly include Time and Space complexity at top of each file
-     def __init__(self):
-
-     def isEmpty(self):
+
+    def __init__(self):
+        self.ls=[]
+
+    def isEmpty(self):
+        return len(self.ls)==0
 
-     def push(self, item):
+    def push(self, item):
+        self.ls.append(item)
 
-     def pop(self):
-
+    def pop(self):
+        if self.isEmpty():
+            return None
+        return self.ls.pop()
 
-     def peek(self):
+    def peek(self):
+        if len(self.ls)>0:
+          return self.ls[-1]
+        return None
 
-     def size(self):
+    def size(self):
+        return len(self.ls)
 
-     def show(self):
+    def show(self):
+        return self.ls
 
 
 s = myStack()

Exercise_2.py

@@ -6,10 +6,20 @@ def __init__(self, data):
 
 class Stack:
     def __init__(self):
+        self.head=Node(-1)
 
     def push(self, data):
-
+       node=Node(data)
+       node.next=self.head.next
+       self.head.next=node
     def pop(self):
+        if self.head.next is None:
+            return None
+        q=self.head.next
+        self.head.next=q.next
+        return q.data
+
+
 
 a_stack = Stack()
 while True:

Exercise_3.py

@@ -3,6 +3,8 @@ 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 +19,42 @@ def append(self, data):
         Insert a new element at the end of the list.
         Takes O(n) time.
         """
+        if self.head is None:
+            self.head = ListNode(data)
+            return
+        curr=self.head
+        while curr.next!=None:
+            curr=curr.next
+        curr.next=ListNode(data)
 
     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!=None:
+            if curr.data==key:
+                return key
+            curr = curr.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
+        curr = self.head
+        while curr.next is not None:
+            if curr.next.data == key:
+                curr.next = curr.next.next
+                return
+            curr = curr.next