[leetcode]Climbing Stairs

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?

很简单的一维DP问题；

算法思路：当最后一步跨完台阶时候，有两种跨法：一步，或者两步。因此得到状态转移方程：f(n) = f(n - 1) + f(n - 2)；

【注意】直接用递归，会超时。

代码如下：

public class Solution {
    public int climbStairs(int n) {
        if(n <= 0) return 0;
        if(n <= 2) return n;
        int[] hash = new int[n + 1];
        hash[1] = 1;
        hash[2] = 2;
        for(int i = 3; i <= n; i++){
            hash[i] = hash[i - 1] + hash[i - 2];
        }
        return hash[n];
    }
}