Quicksort - conditions that makes it stable

Question

A sorting algorithm is stable if it preserves the relative order of any two elements with equals keys. Under which conditions is quicksort stable?

Quick

Answer 1

You can add these to a list, if that's your approach:

• When the elements have absolute ordering
• When the implementation takes O(N) time to note the relative orderings and restores them after the sort
• When the pivot chosen is ensured to be of a unique key, or the first occurence in the current sublist.