70. 爬楼梯

class Solution {
    public int climbStairs(int n) {
        int[] f = new int[n+1];
       
        if(n == 1 || n==2) return n;
        f[1] = 1;
        f[2] = 2;
        for(int i=3;i<=n;i++){
            f[i] = f[i-1]+f[i-2];
        }
        return f[n];

    }
}