LeetCode 70. Climbing Stairs（爬楼梯）

原题网址：https://leetcode.com/problems/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?

方法一：动态规划。

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

方法二：动态规划，优化空间。

public class Solution {
    public int climbStairs(int n) {
        if (n<2) return 1;
        int previous = 1;
        int current = 1;
        for(int i=2; i<=n; i++) {
            int next = previous + current;
            previous = current;
            current = next;
        }
        return current;
    }
}