Stack Data Structure – Important for GATE CSE 2026
Stack is the most important Data Structure for GATE CSE. Every year 2-3 questions are asked from Stack.
What is Stack?
Stack follows LIFO – Last In First Out. Means jo last me aayega wo pehle niklega. Example: Plates ka stack.
Main Operations:
push(x) - Add element on top
pop() - Remove top element
peek() - See top element
isEmpty() - Check if stack empty
Stack Implementation in Python for GATE
stack = []
stack.append(10) # push 10
stack.append(20) # push 20
print(stack.pop()) # Output: 20 - LIFO
GATE Previous Year Questions on Stack
Q1. GATE CSE 2020 – Infix to Postfix
Q: Convert a + b * c to postfix?
Ans: a b c * +
Logic: * has higher priority than +
Q2. GATE CSE 2018 – Stack Push Pop Sequence
Push 1, Push 2, Pop, Push 3, Pop, Pop
What is pop order?
Ans: 2, 3, 1. Because LIFO.
Q3. GATE CSE 2019 – Stack using Queue
Q: How to implement stack using 2 queues?
Ans: For push O(n), for pop O(1) or vice versa. This is very important for GATE.
Q4. Time Complexity – GATE Favorite
Q: Time complexity of push and pop in stack?
Ans: O(1) – Constant time.
Applications of Stack in GATE
- 1. Infix to Postfix conversion
- 2. Evaluation of Postfix expression
- 3. Recursion – Function call stack
- 4. Undo operation
- 5. Browser back button
Conclusion
Stack = LIFO, O(1) push/pop. Practice infix to postfix daily. This topic will give you 2 marks in GATE 2026.
Next Post: Queue Data Structure GATE PYQ.