Challenge: Implement Merge Sort

The mergeSort() function should recursively sort the subarray $array[p \cdots r]$ , i.e., after calling mergeSort(array, p, r) the elements from index p to index r of array should be sorted in ascending order.

To remind you of the merge sort algorithm:

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

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

Use merge(array, p, q, r) to merge sorted sub arrays $array[p \cdots q]$ and $array[q+1 \cdots r]$.