class Solution {
public:
    int Fibonacci(int n) {
        if(n == 0)  return 0;
        if(n == 1)  return 1;
        int a = 0, b = 1;
        for(int i = 2; i <= n; i ++ ){
            int tmp = a + b;
            a = b, b = tmp;
        }
        return b;
    }
};