Stack.py

class Node:
	def __init__(self,data):
		self.data = data
		self.next = None
	
class Stack:
	# head is default NULL
	def __init__(self):
		self.head = None

	# Checks if stack is empty
	def isempty(self):
		if self.head == None:
			return True
		else:
			return False
	
	# Method to add data to the stack
	# adds to the start of the stack
	def push(self,data):
		if self.head == None:
			self.head=Node(data)
			
		else:
			newnode = Node(data)
			newnode.next = self.head
			self.head = newnode
	
	# Remove element that is the current head (start of the stack)
	def pop(self):
		if self.isempty():
			return None
			
		else:
			# Removes the head node and makes
			#the preceding one the new head
			poppednode = self.head
			self.head = self.head.next
			poppednode.next = None
			return poppednode.data
	
	# Returns the head node data
	def peek(self):
		if self.isempty():
			return None
			
		else:
			return self.head.data
	
	# Prints out the stack	
	def display(self):
		iternode = self.head
		if self.isempty():
			print("Stack Underflow")
		
		else:
			while(iternode != None):
				print(iternode.data,"->",end = " ")
				iternode = iternode.next
			return
		
# Stack Implementation :
MyStack = Stack()

MyStack.push(11)
MyStack.push(22)
MyStack.push(33)
MyStack.push(44)

# Display stack elements
MyStack.display()

# Print top element of stack
print("\nTop element is ",MyStack.peek())

# Delete top elements of stack
MyStack.pop()
MyStack.pop()

# Display stack elements
MyStack.display()

# Print top element of stack
print("\nTop element is ", MyStack.peek())