Quick Sort — Why Your Programming Language Uses This Algorithm for .sort()

Every time you call .sort() in JavaScript,
Python, or Java — Quick Sort is running
under the hood.

Not Bubble Sort. Not Merge Sort. Quick Sort.

Why Quick Sort?

It combines two things perfectly:

• O(n log n) average performance
• In-place sorting — no extra memory needed

Merge Sort also achieves O(n log n) but
requires O(n) extra space. Quick Sort only
needs O(log n) stack space for recursion.

The Core Idea — The Pivot

\`javascript
function partition(arr, low, high) {
const pivot = arr[high];
let i = low - 1;

for (let j = low; j < high; j++) {
if (arr[j] <= pivot) {
i++;
[arr[i], arr[j]] = [arr[j], arr[i]];
}
}
[arr[i + 1], arr[high]] = [arr[high], arr[i + 1]];
return i + 1;
}
`\

Pick a pivot. Everything smaller goes left.
Everything larger goes right.
The pivot is now in its permanent position.

The One Weakness

Worst case is O(n²) — when pivot is always
the smallest or largest element.

Modern implementations use randomized pivot
selection to avoid this trap.

Watch It

What's Next

Linked Lists — a completely different
approach to storing data in memory.

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