This kind of question comes up regularly (but not frequently...). Basically, you cannot show the steps of a recursive function, because it won't return before having processed all its stack, and Processing won't update the display before it returned.
By using non-recursive versions of the algorithms. See their quicksort for example: I haven't looked closely at the algorithm, but the usage of S.push() and S.pull() suggests they manage the stack themselves, instead of letting the recursivity doing it with the call stack (stack of parameters / return values).
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