Chalarangelo/30-seconds-of-code

---
title: Quick sort
type: story
language: javascript
tags: [algorithm,array,recursion]
cover: eagle
excerpt: Sort an array of numbers, using the quicksort algorithm.
listed: true
dateModified: 2023-12-16
---

## Definition

[Quicksort](https://en.wikipedia.org/wiki/Quicksort) is a **divide and conquer sorting algorithm**. It first divides a large array into two smaller subarrays and then recursively sorts the subarrays.

## Implementation

- Use recursion.
- Use the spread operator (`...`) to clone the original array, `arr`.
- If the `length` of the array is less than `2`, return the cloned array.
- Use `Math.floor()` to calculate the index of the pivot element.
- Use `Array.prototype.reduce()` and `Array.prototype.push()` to split the array into two subarrays. The first one contains elements smaller than or equal to `pivot` and the second on elements greater than it. Destructure the result into two arrays.
- Recursively call `quickSort()` on the created subarrays.

```js
const quickSort = arr => {
  const a = [...arr];
  if (a.length < 2) return a;
  const pivotIndex = Math.floor(arr.length / 2);
  const pivot = a[pivotIndex];
  const [lo, hi] = a.reduce(
    (acc, val, i) => {
      if (val < pivot || (val === pivot && i != pivotIndex)) {
        acc[0].push(val);
      } else if (val > pivot) {
        acc[1].push(val);
      }
      return acc;
    },
    [[], []]
  );
  return [...quickSort(lo), pivot, ...quickSort(hi)];
};

quickSort([1, 6, 1, 5, 3, 2, 1, 4]); // [1, 1, 1, 2, 3, 4, 5, 6]
```

## Complexity

The algorithm has an [average time complexity](/js/s/big-o-cheatsheet#array-sorting-algorithms) of `O(n log n)`, where `n` is the size of the input array.