【算法】 斐波那契数列

/// <summary>
        /// 递归方式 ： 思路简单但效率比循环方式低，递归次数越多，性能越低
        /// </summary>
        /// <param name="n"></param>
        /// <returns></returns>
        public static int Fn(int n)
        {
            if (n <= 2)
            {
                return 1;
            }
            checked // 检查算术溢出
            {
                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)
        {
            int first = 1;
            int second = 1;
            for (int i = 3; i <= n; i++)
            {
                checked
                {
                    second = first + (first = second);
                }
            }
            return second;
        }