递归-PrintDigit

        static void PrintDigit(double n,int len,int count=-1)
        {
            if (n <0) { n = System.Math.Abs(n); Console.Write("-"); }
            int a = (int)(n * (Math.Pow(10,count)) % 10);
            Console.Write(a);
            if (count == 0) Console.Write(".");
            count++;
            if (count > len) return;
            else
                 PrintDigit(n, len,count);
        }