Quicksort is a divide-and-conquer method for sorting. It works by partitioning an array into two subarrays, then sorting the subarrays independently.

Quicksort is complementary to mergesort: for mergesort, we break the array into two subarrays to be sorted and then combine the ordered subarrays to make the whole ordered array; for quicksort, we rearrange the array such that, when the two subarrays are sorted, the whole array is ordered. In the first instance, we do the two recursive calls before working on the whole array; in the second instance, we do the two recursive calls after working on the whole array. For mergesort, the array is divided in half; for quicksort, the position of the partition depends on the contents of the array.