9.1 A child is running up a staircase with n steps, and can hop either 1 step, 2 steps, or 3 steps at a time. Implement a method to count how many possible ways the child can run up the stairs.

LeetCode上的原题，请参见我之前的博客Climbing Stairs 爬*问题。

class Solution {

public:

    int countWays(int n) {

        vector<int> res(n + , );

        for (int i = ; i <= n; ++i) {

            res[i] = res[i - ] + res[i - ];

        }

        return res.back();

    }

};