Data Structures • Beginner Foundation

Stack (Abstract Data Type)

A stack stores items using LIFO — Last In, First Out. The only access point is the top.

Stacks are one of the most important foundations in computer science. They appear everywhere: function calls (call stack), undo/redo, browser history, expression evaluation, and even algorithms like DFS.

This page teaches the stack mental model and the 3 core operations: push, pop, and peek.

Watch Demo UX/UI • Pedagogy • HCI

Quick Facts

Access pattern

LIFO (Last In, First Out)

Where operations happen

Only at the top of the stack

Common operations

push, pop, peek/top, isEmpty

Time cost (typical)

push/pop/peek are O(1)

Beginner Rules

Push: add a new item on top.
Pop: remove the top item (and return it).
Peek / Top: look at the top item without removing it.
Underflow: popping from an empty stack is an error (or returns nothing).
Only the top is visible: you can’t remove the middle directly.

Why It’s Important

Undo / Redo

Actions are pushed as you work; undo pops the most recent action.

Browser history

Back navigation behaves like a stack: most recent page returns first.

Function calls

The call stack stores return addresses and local variables.

DFS reasoning

DFS backtracking matches stack behavior (explicit stack or recursion).

Demo / Media

Animation

This demo visualizes push, pop, and peek using a clear “top-of-stack” highlight. Watch how the most recently added item is always the first one removed (LIFO).

Common Mistakes (and how to avoid them)

Confusing queue vs stack: stack removes the most recent; queue removes the oldest.
Popping empty stack: always check isEmpty() first (or handle underflow).
Trying to “remove the middle”: stacks are designed for top-only access.

Outcome

Learners can describe LIFO, correctly apply push/pop/peek, and recognize stacks in real applications like undo/redo, browser history, recursion, and DFS.