class Solution {
public:
    int climbStairs(int n) {
        int a = 1, b = 1;
        while (-- n) { //往后走n-1步，最后返回的是b,所以是n-1步
            int c = a + b;
            a = b, b = c;
        }
        return b;
    }
};

class Solution {
public:
    int climbStairs(int n) {
        vector<int> f(n + 1); //这里必须开成n+1,如果是n的话就越界了
        f[0] = 1, f[1] = 1;
        for (int i = 2; i <= n; i ++) f[i] = f[i - 1] + f[i - 2];
        return f[n];
    }
};