LeetCode -- Range Sum Query - Immutable

LeetCode -- Range Sum Query - Immutable


题目描述
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.


传入起始索引，返回数组求和的值。
思路：
使用缓存前n项的和。

public class NumArray {    private int[] _cacheSum ;private int[] _nums;    public NumArray(int[] nums) {_nums = nums;_cacheSum = new int[nums.Length];        var s = 0;        for (var i = 0;i < nums.Length; i++){            s += nums[i];            _cacheSum[i] = s;        }    }    public int SumRange(int i, int j) { if(_nums == null || _nums.Length == 0){return 0;}if(i < 0 || j > _nums.Length - 1){return 0;}return _cacheSum[j] - _cacheSum[i] + _nums[i];    }}