Stable vs. Non-Stable Sorting.

A stable sort is a sort algorithm that performs the same amount of work regardless of the data being sorted. The amount of work a non-stable sort performs, on the other hand, can vary significantly depending on the arrangement of data. This translates to stable sorts having identical best case, worst case, and average case complexity. The worst case complexity of a non-stable sort can be an entirely different runtime class than its best or average case.