using System;

​

public class Program

{

    public static void Main()

    {

        long id = 1234567890;

        var encoded = AlphabetTest.Encode(id);

        Console.WriteLine(id + " : " + encoded);

        //check to make sure

        for (var i = 0; i < 10000000; i++) 

            {

                if (AlphabetTest.Decode(AlphabetTest.Encode(i)) != i) 

                {

                    System.Console.WriteLine("{0} is not {1}", AlphabetTest.Encode(i), i);

                    break;

                }

            }

    }

    public class AlphabetTest 

    {

        public static readonly string Alphabet = "bcdefghjklmnopqrstvwxyzBCDEFGHJKLMNOPQRSTVWXYZ0123456789";

        public static readonly long Base = Alphabet.Length;

​

        public static string Encode(long i)

        {

            if (i == 0) return Alphabet[0].ToString();

​

            var s = string.Empty;

​

            while (i > 0)

            {  

                s += Alphabet[(int)(i % Base)];

                i = i / Base;

            }

​

            return string.Join(string.Empty, ReverseString(s));

        }

​

        public static long Decode(string s)

        {

            long i = 0;

​

            foreach (var c in s)

            {

                i = (i * Base) + Alphabet.IndexOf(c);

            }

​

            return i;

        }

    }

    public static string ReverseString(string s)

    {

        char[] arr = s.ToCharArray();

        Array.Reverse(arr);

        return new string(arr);

    }

}