heap sort is the other (and by far the most popular) in-place non-recursive sorting algorithm used in this test. Heap sort is not the fastest possible in all (nor in most) cases, but it s the de-facto sorting algorithm when one wants to make sure that the sorting will not take longer than O(nlogn). Heap sort is generally appreciated because it is trustworthy: There aren t any "pathological" cases which would cause it to be unacceptably slow. Besides that, sorting in-place and in a non-recursive way also makes sure that it will not take extra memory, which is often a nice feature.