Stack Data Structure

Question 1

Which element is removed first in a Stack?

Accepted Answer

The most recently added element — Stacks follow the Last-In, First-Out (LIFO) principle.

Question 2

A stack follows which ordering discipline?

Accepted Answer

Last-in, first-out ordering — A stack is Last-In-First-Out: the most recently pushed element is the first one popped — like a stack of plates.

Question 3

What is the time complexity of push, pop, and peek on a stack?

Accepted Answer

O(1) — All three operate only at the top, so they are O(1) (amortized O(1) for a dynamic-array-backed stack on push).

Question 4

Which classic problem is naturally solved with a stack?

Accepted Answer

Matching balanced parentheses — Push opening brackets; on a closing bracket, pop and check it matches. An empty stack at the end means balanced — a canonical stack application.

Question 5

Depth-First Search (DFS) uses which structure (explicitly or via recursion)?

Accepted Answer

A stack — DFS explores as deep as possible before backtracking; an explicit stack (or the call stack in recursion) provides exactly that LIFO behavior.

Question 6

The function 'call stack' relates to recursion because…

Accepted Answer

Each call pushes a frame — Every function invocation pushes an activation record; returns pop it. Excessive recursion depth exhausts this stack — a 'stack overflow'.

Question 7

Evaluating a postfix (Reverse Polish) expression uses a stack by…

Accepted Answer

Pushing operands, popping to apply each operator — Scan tokens: push numbers; for an operator pop the needed operands, compute, and push the result. The final stack value is the answer — O(n).

Question 8

A 'min-stack' supporting getMin() in O(1) is implemented by…

Accepted Answer

Storing the running minimum alongside each push — Push the current minimum with each value (or maintain an auxiliary stack of minima) so the minimum is always available at the top in O(1).

Question 9

When implemented with a dynamic array, push is…

Accepted Answer

Amortized O(1) — Most pushes are O(1); when capacity is exceeded the array doubles (O(n)), but amortized over many pushes it averages O(1).

Question 10

'Stack overflow' at runtime is typically caused by…

Accepted Answer

Unbounded, too-deep recursion depth — Each recursive call consumes stack space; runaway or excessively deep recursion overflows the fixed-size call stack.

Stack (LIFO)

Stack Structure

Last in, first out

Test your knowledge

Which element is removed first in a Stack?

Write the code

Valid Parentheses

Example Cases

The short version

In more depth

Last In, First Out (LIFO)

When to reach for this

Interview Questions

References & Further Reading

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