LeetCode_Climbing Stairs

 1 class Solution {

 2 public:

 3     int climbStairs(int n) {

 4         // Start typing your C/C++ solution below

 5         // DO NOT write int main() function

 6         if(n ==0) return 0;

 7         if(n == 1) return 1;

 8         if(n == 2) return 2;

 9         int a1 = 1;

10         int a2 = 2;

11         int a3;

12         for(int i =3;i<=n; i++)

13         {    

14             a3 = a1+ a2;

15             a1 = a2;

16             a2 = a3;

17         }

18         

19         return a3;

20     }

21 };