[LeetCode] Climbing Stairs

Problem:

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?

Solution:O(n)

public class Solution {
    public int climbStairs(int n) {
        if(n<3)
            return n;
        int l1 = 1;
        int l2 = 2;
        int ret = 0;
        for(int i=3; i<=n;i++){
           ret = l1+l2;
           l1 = l2;
           l2 = ret;
        }
        return ret;
    }
}