Tutorial in Chinese and the code here

I'll do my best to explain in English.

First we know if we only have to choose a number x , the answer will be the median.
The only property we need to know for this problem is the best choice of x and y must separate the array into two consecutive part: 1 ~ M for x and M+1 ~ n for y.
What we should do is enumerate M and calculate the cost of two independent parts in O(1) for each M.

Hope it helps :).