Before we go onto the right hand side partition, let us look at how we have divided the problem so far.

Each time we looked at a new left partition we have been doing the exact same algorithm again. This is known as recursion. We needed three levels of recursion before we found we could go no further. Once this happens we must return to the level above and attempt the right hand side. As you can see the right partition at level 2 has only one element. As such we know that this is sorted. We then look at the right partition at level one. This is not sorted so we again apply the quicksort algorithm.

There is nothing to sort on the left hand partition (remember that you always work on the left partition first). 64 is now sorted so we look again at the right hand partition. The pivot will be 99 so our left hand partition will contain 73 and 80 while the right hand partition will contain nothing. Finally the left hand partition will be considered and basically left alone as there is nothing left to move. The final result is -