Day 32 – Evening Session: DS – Stack (PUSH, POP, Applications) – ECET 2026

In ECET 2026 Computer Science, Stack is a very important data structure. Questions are frequently asked from operations (PUSH & POP) and applications. Mastering this topic ensures at least 2–3 marks in the exam.

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⚙️ Concept Notes – Stack

• Definition: A stack is a linear data structure which follows LIFO (Last In, First Out) principle.
• Analogy: Like a stack of plates – the last plate placed on the top is the first one removed.
• Basic Operations:
  • PUSH → Insert element into stack (on top).
  • POP → Remove element from stack (from top).
  • PEEK / TOP → View the top element without removing.

👉 Representation:

• Can be implemented using arrays or linked lists.

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🔢 Operations with Examples

1️⃣ PUSH Operation

• Adds an element to the top of stack.
• If stack is full → Stack Overflow.

Example:
Initial Stack: [10, 20, 30] (top = 30)
PUSH(40) → [10, 20, 30, 40] (top = 40)

2️⃣ POP Operation

• Removes element from the top of stack.
• If stack is empty → Stack Underflow.

Example:
Stack: [10, 20, 30, 40] (top = 40)
POP() → removes 40 → [10, 20, 30] (top = 30)

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🛠️ Applications of Stack

1. Expression Conversion:
   • Infix → Postfix, Prefix → Infix conversions.
   • Example: (A + B) * C → AB + C * (Postfix).
2. Expression Evaluation:
   • Postfix expressions are evaluated using stack.
   • Example: Postfix: 23*54*+ → Result = 26.
3. Function Call Management:
   • Recursion uses stack for maintaining function calls.
4. Undo / Redo in Editors:
   • Operations are stored in stack.
5. Backtracking:
   • Used in puzzles like maze solving, N-Queens problem.
6. Parenthesis Matching:
   • To check balanced brackets { [ ( ) ] }.

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⚙️ Formulas (for Array Implementation of Stack)

• PUSH (item):

• POP():

10 Most Expected MCQs – ECET 2026

Q1. A stack works on which principle?
A) FIFO
B) LIFO
C) Priority
D) Random

Q2. PUSH operation is performed on:
A) Bottom of stack
B) Top of stack
C) Middle of stack
D) None

Q3. When stack is full, it is called:
A) Underflow
B) Overflow
C) Exhaustion
D) Termination

Q4. When stack is empty and POP is performed, it is:
A) Overflow
B) Underflow
C) Both
D) None

Q5. Which of the following is not an application of stack?
A) Expression evaluation
B) Recursion
C) Parenthesis checking
D) Sorting using Merge Sort

Q6. Infix expression (A + B) * C in postfix is:
A) ABC*+
B) AB+C*
C) AB*C+
D) A+BC*

Q7. Which of the following uses stack?
A) Undo in editor
B) Function calls
C) Balancing brackets
D) All of these

Q8. The maximum number of elements in stack depends on:
A) Size of array
B) Number of functions
C) Compiler
D) Recursion depth only

Q9. Time complexity of PUSH and POP is:
A) O(n)
B) O(1)
C) O(log n)
D) O(n log n)

Q10. In array implementation of stack, initial value of TOP is:
A) 0
B) -1
C) 1
D) NULL

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✅ Answer Key

Q.No	Answer
Q1	B
Q2	B
Q3	B
Q4	B
Q5	D
Q6	B
Q7	D
Q8	A
Q9	B
Q10	B

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🧠 Explanations

• Q1 → B: Stack follows LIFO principle.
• Q2 → B: Elements are added at the top.
• Q3 → B: Full stack → Overflow.
• Q4 → B: Empty stack + POP → Underflow.
• Q5 → D: Merge Sort uses divide-and-conquer, not stack.
• Q6 → B: (A+B)*C → AB+C*.
• Q7 → D: Undo, recursion, balancing brackets all use stack.
• Q8 → A: Array size limits stack capacity.
• Q9 → B: Both PUSH and POP are O(1).
• Q10 → B: Initially, TOP = -1 (empty stack).

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🎯 Why This Practice Matters

Stack is a core topic in Data Structures. In ECET exams:

• 2–3 MCQs are usually asked on stack.
• Covers both theory + implementation.
• Also important for coding interviews.

By practicing operations and applications, you’ll score guaranteed marks.

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