[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 res(n + 1, 1);

for (int i = 2; i <= n; ++i) {

res[i] = res[i - 1] + res[i - 2];

}

return res.back();

}

};

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