technically quick sort needs O(log N) additional space and it is still considered in-place. I guess the threshold for in-place-ness would be less than linear additional space?
quicksort is not stable. the disadvantage of classic mergesort is that it needs to copy elements out of the input array, although there exist (slower) in-place variations.
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The term "in-place" almost always means O(1) additional space, whereas this uses O(n) additional space.
technically quick sort needs O(log N) additional space and it is still considered in-place. I guess the threshold for in-place-ness would be less than linear additional space?
quicksort is not stable. the disadvantage of classic mergesort is that it needs to copy elements out of the input array, although there exist (slower) in-place variations.