Leetcode Range Sum Query - Immutable

题意：给出一数组，求i到j之间的和。

题意：记录开始位置到各位置的和，查询时，减去开头部分的和。

class NumArray {
public:
    NumArray(vector<int> &nums) {
        totalSum.push_back(0);
        int sum = 0;
        for(int i = 0; i < nums.size(); ++ i) {
            sum += nums[i];
            totalSum.push_back(sum);
        }
    }
    vector<int> totalSum;
    int sumRange(int i, int j) {
        return totalSum[j + 1] - totalSum[i];
    }
};


// Your NumArray object will be instantiated and called as such:
// NumArray numArray(nums);
// numArray.sumRange(0, 1);
// numArray.sumRange(1, 2);