求反复各位数之和

反复各位数之和 直到其和为一位为止，返回这个数字

ie.   5472576 => 5 +4 +7+2+5+7+6 =36 => 3+6 =9 返回9
  

class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine(Fn(5472576));
            Console.ReadLine();
        }

        private static int AddAllNumber(int num)
        {
            int count = 0;
            while (num!=0)
            {
                count += num % 10;
                num = num / 10;  
            }
            return count;  
        }

        private static int Fn(int num)
        {
            while (num > 9)
            {
                num = AddAllNumber(num);
            }
            return num;
        }
    }