Challenge: Implement Quicksort

The quickSort function should recursively sort the subarray $array[p \cdots r]$.

• If the subarray has size $0$ or $1$, then it's already sorted, and so nothing needs to be done.

• Otherwise, quickSort uses divide-and-conquer to sort the subarray.

The divide step should partition the array, the conquer step should recursively quicksort the partitioned subarrays, and the combine step should do nothing.