Consider the Mergesort algorithm on inputs of size $n = 2^k$. Normally, this algorithm would have a recursion depth of $k$. Suppose that we modify the algorithm so that after $k/2$ levels of recursion, it switches over to insertion sort. What is the running time of this modified algorithm?
I know that MergeSort has worst-case runtime of $O(n \log n)$ and Insertion Sort has worst-case runtime of $O(n^2)$, but I'm not sure how these bounds will be affected within the problem above.