Class 12 Computer Science Chapter 3: Stack – NCERT Solutions & Key Answers

1. State TRUE or FALSE:

(a) Stack is a linear data structure

(b) Stack does not follow LIFO rule

(c) PUSH operation may result into underflow condition

(d) In POSTFIX notation for expression, operators are placed after operands

Answer:
(a) True

(b) False

(c) False

(d) True

2. Find the output of the following code:

(a)

result = 0
numberList = [10, 20, 30]
numberList.append(40)
result = result + numberList.pop()
result = result + numberList.pop()
print("Result =", result)

Answer: Result = 70

(b)

answer = []
output = ''
answer.append('T')
answer.append('A')
answer.append('M')
ch = answer.pop()
output = output + ch
ch = answer.pop()
output = output + ch
ch = answer.pop()
output = output + ch
print("Result =", output)

Answer: Result = MAT

3. Write a program to reverse a string using stack.

Answer:

def push(stack, item):
    stack.append(item)
def pop(stack):
    if stack == []:
        return
    return stack.pop()
def reverse(string):
    n = len(string)
    stack = []
    for i in range(n):
        push(stack, string[i])
    string = ""
    for i in range(n):
        string += pop(stack)
    return string
string = input("Enter a string: ")
print("String:", string)
reversedStr = reverse(string)
print("Reversed String:", reversedStr)

4. For the arithmetic expression: ((2+3)*(4/2))+2 show the parentheses-matching process step by step using a stack.

Answer:

• Scan from left to right. For each opening parenthesis '(', push onto the stack. For each closing parenthesis ')', pop from the stack.
• If stack is empty after entire scan and every ')' matched an '(', then the parentheses are balanced.

5. Evaluate the following postfix expressions (given A=3, B=5, C=1, D=4; show stack after each step):

(a) A B + C *

Answer:

A = 3, B = 5, C = 1

• Step-by-step:
  Push 3 → [3]
  Push 5 → [3,5]
  Pop 5, 3, add → 8
  Push 8 → [8]
  Push 1 → [8,1]
  Pop 1, 8, multiply → 8
  Final Stack: [8], Answer = 8
  
  

Answer:

• Step-by-step:
  Push 3 → [3]
  Push 5 → [3,5]
  Pop 5, 3, multiply → 15
  Push 15 → [15]
  Push 1 → [15,1]
  Pop 1, 15, divide → 15/1 = 15
  Push 15 → [15]
  Push 4 → [15,4]
  Pop 4, 15, multiply → 60
  Final Stack: [60], Answer = 60

6. Convert the following infix expressions to postfix notation, showing stack and string at each step:

(a) A + B - C * D

Answer: AB+CD*-

(b) A * (( C + D ) / E )

Answer: ACD+E/*

7. Write a program to create a Stack for storing only odd numbers out of all the numbers entered by the user. Display the content of the Stack along with the largest odd number in the Stack. (Hint: Pop all elements, maintain the largest so far.)

Answer:

def push(stack, item):
    stack.append(item)
def pop(stack):
    if stack == []:
        return
    return stack.pop()
def oddStack(num):
    if num % 2 == 1:
        push(stack, num)
def GetLargest(stack):
    elem = pop(stack)
    large = elem
    while elem != None:
        if large < elem:
            large = elem
        elem = pop(stack)
    return large
n = int(input("How many numbers?"))
stack = []
for i in range(n):
    number = int(input("Enter number:"))
    oddStack(number)
print("Stack created is", stack)
bigNum = GetLargest(stack)
print("Largest number in stack", bigNum)

Detailed NCERT Solutions for Class 12 Computer Science Chapter 3 – Stack

Understanding the basics of Stack data structure is crucial in Class 12 Computer Science. With clear concepts and practice, you can apply the LIFO principle and confidently solve Python stack questions in your exams.

The NCERT Solutions Class 12 Computer Science Chapter 3 Stack for 2025-26 guide you through stack operations and their applications. Use algorithm examples and Python implementation tips to strengthen your exam preparation and coding skills.

Regular revision and solving exercise-based questions from this chapter will help you master stack concepts. Focus on the applications, and you’ll be ready to score high in your board exams and future programming challenges.