Climbing stairs--leetcode

Question:

You are climbing a stair case. It takes n steps to reach to the top.

Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb to the top?

Answer:

class Solution {
public:
    int climbStairs(int n) {
        if(n==0) return 0;
        int fn_2 = 1, fn_1=2, fn;
        if(n==1) return fn_2;
        if(n==2) return fn_1;
                                              //if(n>=3)
        for(int i=3;i<=n;++i){
            fn = fn_1 + fn_2;
            fn_2 = fn_1;
            fn_1 = fn;
        }
        return fn_1;
    }

};