[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。

 1 class Solution {
 2 public:
 3     int climbStairs(int n) {
 4         if (n==1) return 1;
 5         if (n==2) return 2;
 6         int a = 1, b = 2;
 7         int c;
 8         for (int i = 3; i <= n; i++) {
 9             c = a + b;
10             a = b;
11             b = c;
12         }
13         return c;
14     }
15 };

第二遍。

1 class Solution {
2 public:
3     int climbStairs(int n) {
4         vector<int> dp(3, 1);
5         for (int i = 2; i <= n; ++i)
6             dp[i % 3] = dp[(i-1) % 3] + dp[(i-2) % 3];
7         return dp[n % 3];
8     }
9 };