Quicksort Revisited - Verifying Alternative Versions of Quicksort

We verify the correctness of a recursive version of Tony Hoare's $$\texttt {quicksort}$$quicksort algorithm using the Hoare-logic based verification tool Dafny. We then develop a non-standard, iterative version which is based on a stack of pivot-locations rather than the standard stack of ranges. We outline an incomplete Dafny proof for the latter.