[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?

 

其实就是斐波那契数列，递推求解便知。

 

class Solution {
public:
    int climbStairs(int n) {
        if (n==1) return 1;
        if (n==2) return 2;
        int a = 1, b = 2;
        int c;
        for (int i = 3; i <= n; i++) {
            c = a + b;
            a = b;
            b = c;
        }
        return c;
    }
};

 第二遍。

class Solution {
public:
    int climbStairs(int n) {
        vector<int> dp(3, 1);
        for (int i = 2; i <= n; ++i)
            dp[i % 3] = dp[(i-1) % 3] + dp[(i-2) % 3];
        return dp[n % 3];
    }
};