[LeetCode] 303. Range Sum Query - Immutable

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题：https://leetcode.com/problems/range-sum-query-immutable/description/

题目

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.

题目大意

实现方法，求区间 i ~ j 的和，i、j包含。

解题思路

求区间 i ~ j 的和，可以转换为 sum[j + 1] - sum[i]，其中 sum[i] 为 0 ~ i - 1 的和。

求解sum[j] = sum[j-1] + nums[j-1]

class NumArray {int[] sums;public NumArray(int[] nums) {sums = new int[nums.length+1];sums[0] = 0;for(int i = 1;i<=nums.length;i++){sums[i] = sums[i-1] + nums[i-1];}}public int sumRange(int i, int j) {return sums[j+1] - sums[i];}
}/*** Your NumArray object will be instantiated and called as such:* NumArray obj = new NumArray(nums);* int param_1 = obj.sumRange(i,j);*/

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