Stability of quicksort partitioning approach

Question

Does the following Quicksort partitioning algorithm result in a stable sort (i.e. does it maintain the relative position of elements with equal values):

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Answer 1

A sort is stable when the original order of similar elements doesn't change. Your algorithm isn't stable since it swaps equal elements.

If it didn't, then it still wouldn't be stable:

( 1, 5, 2, 5, 3 )

You have two elements with the sort key "5". If you compare element #2 (5) and #5 (3) for some reason, then the 5 would be swapped with 3, thereby violating the contract of a stable sort. This means that carefully choosing the pivot element doesn't help, you must also make sure that the copying of elements between the partitions never swaps the original order.