ClimbingStairs爬楼梯问题，每次可以走1或2步，爬上n层楼梯总方法（变相fibonacci）

 1 class Solution {
 2 public:
 3     int climbStairs(int n) {
 4         if(n <0)
 5             return -1;
 6         int res[] = {0,1};
 7         if(n<2)
 8             return res[n];
 9             
10         int fib1 = 0;
11         int fib2 = 1;
12         
13         int result = 1;
14         
15         for(int i = 1 ; i <= n ; i++){
16             result = fib1 + fib2;
17             fib1 = fib2;
18             fib2 = result;
19         }
20         
21         return result;
22     }
23 };