LeetCode_Climbing Stairs

class Solution {
public:
    int climbStairs(int n) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if(n ==0) return 0;
        if(n == 1) return 1;
        if(n == 2) return 2;
        int a1 = 1;
        int a2 = 2;
        int a3;
        for(int i =3;i<=n; i++)
        {    
            a3 = a1+ a2;
            a1 = a2;
            a2 = a3;
        }
        
        return a3;
    }
};