Sorting and Searching

O(n log n) in best, average, and worst cases; O(n) extra space for arrays.

O(n²), when pivots are consistently the smallest or largest element (e.g., already-sorted input with a first-element pivot).

Randomized pivot selection or median-of-three, and introsort switches to heapsort when recursion gets too deep.

Equal keys keep their original relative order after sorting.

Merge sort, insertion sort, bubble sort, counting sort, and Timsort; standard quicksort and heapsort are not.

It runs in O(n) on nearly sorted input, has low overhead, and is used for small subarrays inside hybrid sorts like Timsort.