[CareerCup] 9.1 Climbing Staircase 爬楼梯

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 爬梯子问题。

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();
    }
};