327. Count of Range Sum

1. 题目表述

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.

Note:A naive algorithm of O(n2) is trivial. You MUST do better than that.

Example:

Given nums = [-2, 5, -1], lower = -2, upper = 2, 
Return 3. 
The three ranges are : [0, 0], [2, 2], [0, 2] and their respective sums are: -2, -1, 2. 

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2. 算法分析