Data Structure: Stack — Part 2: Applications and Board-Style Programs: visual lesson, CBSE Class 12 Computer Science

Lesson

Stacks at work

Part 2 — applications of stacks and the push/pop programs asked in board papers.

In this lesson you will

Use a stack to reverse data, check brackets and evaluate postfix expressions
Explain how the call stack, undo and back buttons follow LIFO
Write push and pop functions that move items meeting a condition into a stack and display them

Part 2 at a glance

Concept

Whenever the most recent item must be handled first, a stack fits.

Table of stack applications: reversing text pushes characters, bracket checking pushes opening brackets, function calls push active calls, undo and back push actions or pages, and postfix evaluation pushes operands
Application	What is pushed	Why LIFO fits
Reversing text	Each character	The last character comes out first
Bracket checking	Each opening bracket	A closing bracket must match the latest open one
Function calls	Each active call	The latest call finishes first
Undo / Back	Each action or page	The latest action is undone first
Postfix evaluation	Each operand	An operator uses the two latest operands

Diagram

Popping every letter gives AOG.

Push every character of a word, then pop them all:

stk = []
for ch in "GOA":
    stk.append(ch)
rev = ""
while stk:
    rev += stk.pop()
print(rev)