Leetcode 303: Range Sum Query - Immutable

转载于 2018-02-11 09:31:00 发布 · 55 阅读

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原文链接：http://www.cnblogs.com/liangmou/p/8440478.html

本文介绍了一种利用前缀和数组实现快速区间求和的方法。通过预处理整数数组生成前缀和数组，可以高效地计算任意指定区间内的元素总和。这种方法适用于数组固定不变且区间求和操作频繁的场景。

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Given an integer array nums, find the sum of the elements between indices i and j (i ≤ j), inclusive.

Example:

Given nums = [-2, 0, 3, -5, 2, -1]

sumRange(0, 2) -> 1
sumRange(2, 5) -> -1
sumRange(0, 5) -> -3

Note:

You may assume that the array does not change.
There are many calls to sumRange function.

 1 public class NumArray {
 2     private int[] presum = null;
 3     
 4     public NumArray(int[] nums) {
 5         this.presum = new int[nums.Length];
 6         
 7         int pre = 0;
 8         
 9         for (int i = 0; i < presum.Length; i++)
10         {
11             presum[i] = pre + nums[i];
12             pre = presum[i];
13         }
14     }
15     
16     public int SumRange(int i, int j) {
17         return presum[j] - (i == 0 ? 0 : presum[i - 1]);
18     }
19 }
20 
21 /**
22  * Your NumArray object will be instantiated and called as such:
23  * NumArray obj = new NumArray(nums);
24  * int param_1 = obj.SumRange(i,j);
25  */