Fix partition function

[Gothor]

It only fixes the partition function, you still would have to find a way to animate it.

[shiffman]

Where's my facepalm emoji. . I am happy to merge this but I am going to play around with it a bit more (after some sleep) and see where I went wrong.

[Gothor]

Not sure that's the good place for it (please excuse me if it's not the case), but here is me trying a little explanation.

left <= right doesn't work, because it would cause left to increment when it's equal to right. Making left = right + 1. But it may be array[right] > pivot if there's been a swap before. (But later, you used to swap left and the pivot.)

left < hi - 1 doesn't work if the last value is the only one higher than the pivot. Because it would end with left = right = hi - 1, and still, the last swap would have to occur, since it may be array[hi-1] > pivot. So it seems better to check the values.

Then, if this last swap happens, you indeed should return left.
But if it doesn't, you should return hi as the pivot didn't move.