LeetCode : Range Sum Query

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.

class NumArray {
public:
    vector<int>res(0);
    NumArray(vector<int> nums){
        int sum = 0;

        for(auto i:nums){
            sum+=i;
            res.push_back(sum);
        }
    }
    int sumRange(int i, int j) {
        return res[j+1] - res[i];
    }
};