BEGINNER · 8 MIN · FREE LESSON

Stack

Last in. First out.

Size 8

Empty

default
? comparing
↔ moving
✓ visited
Q queued
✓ done

An empty stack is ready for its first value.

Code language

Speed

1 / 26

Live variables

top: -1

Operation counters

comparisons: 0
swaps: 0
reads: 0
writes: 0

Complexity · O(1) per push/pop

best: O(1) per push/pop
average: O(1) per push/pop
worst: O(1) per push/pop
space: O(n)

Only the top is accessed. Holding n values requires n storage slots.

n=8

n=16

n=32

n=64

Measured reads + comparisons, seed 42. Plotted sample sizes respect each lesson’s explicit input limits.

Keyboard: Space play/pause · ←/→ step · R restart · 1–5 speed. Reduced motion disables transitions.

Stack

Hook

A stack of plates only lets you take the plate on top.

Watch

Use the player above to run, pause, scrub, or step backwards through a deterministic trace.

How it works

Push adds a value at the top.
Peek reads the top without removing it.
Pop removes the most recently added value.

Try it

Switch to Try mode and predict each operation before revealing its result.

Complexity

Push and pop touch only one end. Use best, average and worst presets to compare measured counters. The chart plots actual reads and comparisons for n = 8, 16, 32 and 64.

Code

Choose JavaScript, Python, Java or C++ in the synchronized code panel.

When to use

Undo history, expression parsing and depth-first traversal.

Interview pitfalls

Always check for an empty stack before a pop or peek.

Quiz

Complete the three questions below, then use the module navigation to choose your next lesson.

MAKE IT STICK

Quick knowledge check

Progress saves on this device and merges with your account after sign-in.

Your notebook

Private lesson notes

0 / 5000 · Saved on this device; synced when signed in.

Loading interactive lesson…

Next: Queue →

Quick knowledge check

Progress saves on this device and merges with your account after sign-in.