最近面试遇到的关于字符串的输出查找题目

   /// <summary>
        /// 输出第一个只出现一次的字母
        /// </summary>
        private static void OutPutTheFirstLetter()
        {
            string a = Console.ReadLine();
            byte[] array = System.Text.Encoding.ASCII.GetBytes(a);
            int b = -1;
            for (int i = 0; i < array.Length; i++)
            {
                for (int j = i + 1; j < array.Length; j++)
                {
                    if (array[i].ToString() == array[j].ToString())
                    {
                        b = i;
                        goto Found;


                    }
                }
            }
        Found:
            if (b == -1)
            {
                Console.WriteLine("未找到");
                Console.ReadKey();
            }
            else
            {
                Console.WriteLine(System.Text.Encoding.ASCII.GetString(array, b, 1));
                Console.ReadKey();
            }

        }
        /// <summary>
        /// 输入字符串 倒序输出
        /// </summary>
        private static void OutPutDesc()
        {
            string a = Console.ReadLine();

            byte[] array = System.Text.Encoding.ASCII.GetBytes(a);
            byte[] arr2 = new byte[array.Length];
            for (int i = 0; i < array.Length; i++)
            {
                arr2[i] = array[array.Length - i - 1];
            }
            Console.WriteLine(System.Text.Encoding.ASCII.GetString(arr2));
            Console.ReadKey();
        }