class Solution {
    public int climbStairs(int n) {
int f1=1;
        int f2=2;
        int f3=3;
        if (n <= 2) {
            return n;
        } else {
            for (int i=3;i<n+1;i++){
                f3=f1+f2;
                f1=f2;
                f2=f3;
            }
        }
        return f3;
    }
}