Hong finds that the kth smallest number problem is too easy. He comes up with a harder problem.
Given N integers A1...An. Let Bk = |Ai - Aj|(1<=i<j<=N), we can get C(n, 2) Bk. Hong wants to know the median of these defferences.
If C(n, 2) is even, the median is defined as the (C(n, 2) / 2)th smallest number.