Given an integer array nums, return the number of range sums that lie in [lower, upper] inclusive.
Range sum S(i, j) is defined as the sum of the elements in nums between indices i and j (i ≤ j), inclusive.
A naive algorithm of O(n2) is trivial. You MUST do better than that.
Given nums = [-2, 5, -1], lower = -2, upper = 2,
The three ranges are : [0, 0], [2, 2], [0, 2] and their respective sums are: -2, -1, 2.