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?

Solution:
step[n]=step[n-1]+step[n-2], Fibonacci sequence
Dynamic programming or iterations

class Solution {
public:
    int climbStairs(int n) {
        if (n<=1)
            return n;
        if (n==2)
            return 2;
        int first = 1;
        int second = 2;
        int res = 0;
        for (int i=3;i<=n;i++)
        {
            res = first+second;
            first = second;
            second = res;
        }
        return res;
    }
};