字符串加密，自己做了，但总有问题

class program
    {
        public static void Main(string[] args)
        {

            string str = aa("Hello3?#@Xyz89");
            Console.WriteLine(str);
            Console.ReadKey();
        }

        public static string aa(string str)
        {
            char[] arrChar = str.ToCharArray();
            for (int i = 0; i < arrChar.Length; i++)
            {
                if (65 <= arrChar[i] && arrChar[i] <= 90)
                {
                    int j = arrChar[i] + 2;
                    if (j > 90)
                    {
                        j -= 26;
                    }
                    arrChar[i] = (char)j;
                }
                else if (97 <= arrChar[i] && arrChar[i] <= 122)
                {
                    int k = arrChar[i] + 2;
                    if (k > 122)
                    {
                        k -= 26;
                    }
                    arrChar[i] = (char)k;
                }
                else if (48 <= arrChar[i] && arrChar[i] <= 57)
                {
                    int n = arrChar[i] + 3;
                    if (n > 57)
                    {
                        n -= 10;
                    }
                    arrChar[i] = (char)n;
                }
            }
            return new string(arrChar);
        }
    }