Suppose in MergeSort we split the given array of size n to two subarrays of size n/4 and size 3n/4 respectively, sort them, and merge the results. Use the recurrence tree method to guess a tight upper bound and a tight lower bound, and prove your guesses are tight and correct. Hint: Sketch a recurrence tree for recursive calls and try to calculate the totla cost. Such a tree is not balanced, but you can formulate the longest path (from the root to a leaf) and the shortest path on the tree. Use them to guess an upper bound and a lower bound. If you guessed correctly, you should be able to show that both bounds coincide asymptotically resulting in (nlogn) running time.