leetcode 303. Range Sum Query - Immutable 【easy】

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.

题解：
class NumArray {

public:

vector<int> acArray;

NumArray(vector<int> &nums) {

acArray.clear();

int sum =0;

int len = nums.size();

for (int i=0;i<len;i++){

sum+=nums[i];

acArray.push_back(sum);

}

}

int sumRange(int i, int j) {

return acArray[j]-acArray[i-1];

}

};

思路：

从a=0开始，到a=数组长度-1，分别算出【0，a】的和

【0，j】-【0，i-1】即可得到【i，j】的值