/// <summary>        /// 递归方式        /// </summary>        /// <param name="n"></param>        /// <returns></returns>        public static int Fn(int n)        {            if (Default(n)) return 1;            return Fn(n - 1) + Fn(n - 2);        }        /// <summary>        /// 循环方式        /// </summary>        /// <param name="n"></param>        /// <returns></returns>        public static int FnFor(int n)        {            if (Default(n)) return 1;            int first = 1;            int second = 1;            for (int i = 3; i <= n; i++)            {                second = first + (first = second);            }            return second;        }        /// <summary>        /// 默认处理        /// </summary>        /// <param name="n"></param>        /// <returns></returns>        private static bool Default(int n)        {            if (n <= 0)            {                throw new Exception("输入的数字必须是大于0的整数");            }            return n <= 2;        }