Climbing Stairs

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?

 

public class Solution {
    public int climbStairs(int n) {
        if (n <= 0) {
        	return 0;
        }
        if (n == 1) {
        	return 1;
        } else if (n == 2) {
        	return 2;
        }
        int s1 = 1;
        int s2 = 2;
        int s3 = 1;
        for (int i = 3; i <= n; i++) {
        	s3 = s1+s2;
        	s1 = s2;
        	s2 = s3;
        }
        return s3;
    }
}