class NumArray {
private:
    vector<int> sum; // cache
    // sum[i] 表示 nums[0...(i-1)] 的和

public:
    NumArray(vector<int> nums) {
        sum.resize(nums.size() + 1);  // 注意大小比 nums 大 1
        for(int i = 0; i < nums.size(); i++)
            sum[i + 1] = sum[i] + nums[i];
    }

    int sumRange(int i, int j) {
        return sum[j + 1] - sum[i];
    }
};

/**
 * Your NumArray object will be instantiated and called as such:
 * NumArray obj = new NumArray(nums);
 * int param_1 = obj.sumRange(i,j);
 */