# Python Program to Implement a Stack using Linked List class Node:    def __init__(self, data):       self.data = data       self.next = None class Stack:    def __init__(self):        self.head = None     def push(self, data):        if self.head is None:            self.head = Node(data)        else:            new_node = Node(data)            new_node.next = self.head            self.head = new_node     def pop(self):        if self.head is None:            return None        else:            popped = self.head.data            self.head = self.head.next            return popped a_stack = Stack()while True:    print('push <value>')    print('pop')    print('quit')    do = input('What would you like to do? ').split()     operation = do[0].strip().lower()    if operation == 'push':        a_stack.push(int(do[1]))    elif operation == 'pop':        popped = a_stack.pop()        if popped is None:            print('Stack is empty.')        else:            print('Popped value: ', int(popped))    elif operation == 'quit':        break