BEGINNER · 8 MIN · FREE LESSON

Big-O notation

Count the work, not the clock.

Size 8

default
? comparing
↔ moving
✓ visited
Q queued
✓ done

Count one read for each value in this single-pass sum.

Code language

Speed

1 / 14

Live variables

n: 6
sum: 0

Operation counters

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

Complexity · O(n)

best: O(n)
average: O(n)
worst: O(n)
space: O(1)

This single-pass sum visits each element exactly once, so doubling n doubles its reads.

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.

Big-O notation

Hook

Like counting plates to wash, input size tells us how much work remains.

Watch

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

How it works

Visit each input once.
Increment the reads counter.
Compare operation growth as input grows.

Try it

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

Complexity

A single pass takes linear time. 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

Summing a stream or scanning a list.

Interview pitfalls

Big-O ignores constants; it does not predict exact milliseconds.

Quiz

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

MAKE IT STICK

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Next: Time vs space complexity →