---
title: "quick sort"
url: https://perrotta.dev/2026/01/quick-sort/
last_updated: 2026-01-14
---


First element is the pivot:

```python
def quick_sort(a):
    if len(a) < 2: ## [0, 1]
        return a

    pivot = a[0]
    left = [x for x in a[1:] if x < pivot]
    right = [x for x in a[1:] if x >= pivot]

    left = quick_sort(left)
    right = quick_sort(right)

    return left + [pivot] + right

a = list(range(10))[::-1]

# it's usually easier to do it with a copy
assert quick_sort(a) == list(range(10))
```

With a random pivot:

```
import random

def quick_sort(a):
    if len(a) < 2: ## [0, 1]
        return a

    pivot = random.choice(a)
    a_nopivot = [x for x in a if x != pivot]

    left = [x for x in a_nopivot if x < pivot]
    right = [x for x in a_nopivot if x >= pivot]

    left = quick_sort(left)
    right = quick_sort(right)

    return left + [pivot] + right

a = list(range(10))[::-1]

# it's usually easier to do it with a copy
assert quick_sort(a) == list(range(10))
```