70.爬楼梯

两天宇宙人

2020-02-15

😂自顶向下

class Solution {
public:
    int climbStairs(int n) {
        f.resize(1000);
        f[1] = 1;
        f[2] = 2;
        return helper(n);
    }
    int helper(int n) {
        if(f[n] || n <= 2)
            return f[n];
        f[n] = helper(n - 1) + helper(n-2);
        return f[n]; 
    }
private:
    vector<int> f;
};