Stack — NCERT Solutions

# Program to reverse a string using stack

s = input("Enter a string: ")
stack = []
rev = ""

# push each character onto stack
for ch in s:
    stack.append(ch)

# pop characters to form reversed string
while len(stack) != 0:
    rev = rev + stack.pop()

print("Reversed string is:", rev)

To check matching parentheses, scan the expression from left to right and use a stack.

Expression: ((2+3)*(4/2))+2

At the end, the stack is empty, so all parentheses are properly matched.

Given: A = 3, B = 5, C = 1, D = 4

a) A B + C *

Evaluate from left to right using a stack:

Final result = 8.

b) A B * C / D *

Final result = 60.

a) A + B - C * D

Use operator precedence: * before + and -, and + / - are left associative.

Stepwise postfix conversion:

Equivalent postfix expression: **AB+CD*-**

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

Stepwise postfix conversion:

Equivalent postfix expression: **AC+DE/***

# Stack for storing only odd numbers

stack = []
nums = []

n = int(input("How many numbers? "))
for i in range(n):
    x = int(input("Enter number: "))
    nums.append(x)
    if x % 2 != 0:
        stack.append(x)

print("Stack contents are:")
for i in range(len(stack)-1, -1, -1):
    print(stack[i])

if len(stack) == 0:
    print("No odd numbers in stack")
else:
    largest = stack.pop()
    while len(stack) != 0:
        item = stack.pop()
        if item > largest:
            largest = item
    print("Largest odd number is:", largest)