【leetcode】Climbing Stairs (easy)

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?

跳台阶，之前写过递归的，这次写个非递归的。

到第n层的方式 其实就是 到第n-1层的方式（爬一层）和到第n-2层的方式（爬两层）的和

class Solution {
public:
    int climbStairs(int n) {
        if(n <= 0) return 0;
        else if(n == 1) return 1;
        else if(n == 2) return 2;
        else
        {
            int way1 = 2;
            int way2 = 1;
            int ans = 0;
            for(int i = 3; i <= n; i++)
            {
                ans = way1 + way2;
                way2 = way1;
                way1 = ans;
            }
            return ans;
        }
    }
};