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On average, quicksort will be faster than mergesort because the partitioning process of breaking the set into "smaller" and a "larger" subsets is less computationally expensive than mergesort's generalized merge operation. On the other hand, if the set is pathological, e.g. it's already sorted, Quicksort will degenerate into a O(N^2^N*N) behavior while mergesort will always be O(N*log(N)).
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