Heap sort is "obviously" in-place, and O(n log n) time complexity.

This is also one definition that is sometimes used. It has nothing to do with the algorithm, but more with the interface.

//in-place sortarray = [2,3,1]sort(array)print array  //prints [1,2,3]//not in-place sort:array = [2,3,1]array2 = sorted(array)print array  //prints [2,3,1]. was not sorted in-placeprint array2  //prints [1,2,3]. the sorted array was returned 

The underlying sort could be any sort algo, naturally. Some algos are just better suited for in-place sorting and some are better suited for non-in-place sorting (i.e. they put the sorting results in a separate space anyway, and to be in-place they''d have to copy this data to the original place).