Challenge: Implement Partition

The partition function should partition the subarray $array[p \cdots r]$ so that all elements in $array[p \cdots q-1]$ are less than or equal to $array[q]$ (the pivot) and all elements in $array[q+1 \cdots r]$ are greater than $array[q]$, and it returns the index $q$ of where the pivot ends up.

Use the provided swap() function for swapping.