没啥好说的，递归和动态规划的入门。

开始刷动态规划了。

class Solution {
  public:
    int Fibonacci(int n) {
      int fir = 0, sec = 1, res = 1;
      
      for (int i = 2; i <= n; ++i) {
        res = fir + sec;
        fir = sec;
        sec = res;
      }
      
      return res;
    }
};