$6\mathrm{.}5$ Consider the following recursive mergesort algorithm $($another classic divide and conquer algorithm$).$ Mergesort was first described by John Von Neumann in $1945.$ The basic idea is to divide an unsorted list x of m elements into two sublists of about half the size of the original list. Repeat this operation on each sublist, and continue until we have lists of size $1$ in length. Then starting with sublists of length $1,$merge$$ the two sublists into a single sorted list.