示例：
​
给定 nums = [-2, 0, 3, -5, 2, -1]，求和函数为 sumRange()
​
sumRange(0, 2) -> 1
sumRange(2, 5) -> -1
sumRange(0, 5) -> -3
说明:
​
你可以假设数组不可变。
会多次调用 sumRange 方法。

class NumArray {
private:
    vector<int> num;
public:
    NumArray(vector<int>& nums) {
        num=nums;
    }
​
    int sumRange(int i, int j) {
        return accumulate(num.begin()+i, num.begin()+j+1,0);
    }
};
​
/**
 * Your NumArray object will be instantiated and called as such:
 * NumArray* obj = new NumArray(nums);
 * int param_1 = obj->sumRange(i,j);
 */