Updated runtime

It could be O(MN) or O(MN*something). I cannot prove the latter although
it seems possible. The problem seems to be stemming from how T(m,n) =
O(mn) + sum_i_to_n (O(m/2, i) + O(m/2, n-i)) + T(m/2, k) + T(m/2, n-k);
where k is the chosen value to split at. It seems the subproblems become
vastly less complex and thus wouldn't amount to the same O(MN) value. I
don't know yet. It's hard to evaluate.