Climbing Stairs

爬楼梯的递归版本（在leetcode上超时，在本地测试通过）：

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

爬楼梯的迭代版本：

class Solution {
public:
    int climbStairs(int n) {
        if(n == 1) 
            return 1;
        if(n == 2)
            return 2;
      int forward,backward,tmp;
      forward = 1;
      backward = 2;
      for(int i = 0; i < n -2; i++ ) {
        tmp = forward + backward;
        forward = backward;
        backward = tmp;
      }
      return backward;
      }
};