Leetcode: 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?

算是简单的DP，如果想不到会感觉没思路。 最懒的方法是递归，不过时间过不了。

class Solution {
public:
    int climbStairs(int n) {
        if (n <= 2) {
            return n;
        }
        
        return (climbStairs(n-1) + climbStairs(n-2));
    }
};

改成迭代就可以啦，利用前面计算过的结果。

class Solution {
public:
    int climbStairs(int n) {
        if (n <= 2) {
            return n;
        }
        
        int sn, sn2 = 1, sn1 = 2;
        for (int i = 3; i <= n; ++i) {
            sn = sn2 + sn1;
            sn2 = sn1;
            sn1 = sn;
        }
        
        return sn;
    }
};

=====================第二次=====================

class Solution {
public:
    int climbStairs(int n) {
        if (n <= 2) {
            return n;
        }
        
        int n1 = 1;
        int n2 = 2;
        int ways = 0;
        for (int i = 3; i <= n; ++i) {
            ways = n1 + n2;
            n1 = n2;
            n2 = ways;
        }
        
        return ways;
    }
};