输入: n = 3 输出: 6 示例 2：
​
输入: n = 9 输出: 45

1 <= n <= 10000

class Solution {
vector<int> sums={1,3,6};
public:
    int sumNums(int n) {
        if(n<=sums.size()) return sums[n-1];
        return sumNums(n-1)+n;
    }
};