答案

public class Solution {
    public int climbStairs(int n) {
        if(n<4)
        {
            return n;
        }
        int low=1;
        int up=2;
        int p;
        for(int i=3;i<=n;i++)
        {
            p=low+up;
            low=up;
            up=p;
        }
        return up;
    }
}