Exercise_2.py

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""
Created on Sun Feb  8 16:20:59 2026

@author: rishigoswamy

# Time Complexity:
# push : O(1)  (insert at head)
# pop  : O(1)  (remove from head)
#
# Space Complexity:
# O(n), where n is the number of elements currently in the stack.

"""

class Node:
    def __init__(self, data):
       self.data = data
       self.next = None
 
class Stack:
    def __init__(self):
        self.head = Node(None)
        
    def push(self, data):
        newNode = Node(data)
        if self.head.next:
            newNode.next = self.head.next
        self.head.next = newNode
            
        
    def pop(self):
        if self.head.next:
            val = self.head.next.data
            self.head.next = self.head.next.next
            return val
        else:
            return None
        
        
a_stack = Stack()
while True:
    #Give input as string if getting an EOF error. Give input like "push 10" or "pop"
    print('push <value>')
    print('pop')
    print('quit')
    do = input('What would you like to do? ').split()
    #Give input as string if getting an EOF error. Give input like "push 10" or "pop"
    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