Quicksort is a more efficient sorting algorithm than selection sort, in most cases, and it makes use of recursion.

Recursion means we call a function from within the same function. It’s a very useful practice, sometimes, and this is one of those cases.

I said “in most cases”, because as we’ll see, in the worst case quicksort can take the same time as selection sort: O(n^2). But in the best case scenario, it will run at O(n log n), which is between O(n) and O(n^2).

How does it work? Given an array, we pick an item, called pivot. We then get all the items smaller than the pivot, and the items bigger than the pivot.

Then we run the same operation on the two arrays that contain the smaller and bigger items.

It’s easier to see the code than to describe it:

const quickSort = (originalList) => {
  const list = [...originalList]

  if (list.length < 2) {
    return list
  }

  const pivot = list[0]

  const smaller = list.filter((item) => item < pivot)
  const bigger = list.filter((item) => item > pivot)

  return [...quickSort(smaller), pivot, ...quickSort(bigger)]
}

In this case I chose the pivot to be the first item in the array, but it could also be the item in the middle, for example:

const pivot = list[Math.floor(list.length / 2)]

Notice how we first copy the array, so calling quickSort() does not modify the original array, it just returns a new sorted array:

const a = [1, 6, 3, 4, 5, 1, 0, 4, 8]

console.log(quickSort(a))
//[0, 1, 1, 3, 4, 4, 5, 6, 8]

console.log(a)
//[1, 6, 3, 4, 5, 1, 0, 4, 8]

Lessons in this unit:

0:	Introduction
1:	Linear Search
2:	Binary Search
3:	Bubble Sort
4:	Selection Sort
5:	Merge Sort
6:	▶︎ Quicksort
7:	Algorithm Complexity and Big O Notation