using System;

public class Program

{

    private static string Alphabet =

            "0123456789" +

            "ABCDEFGHIJKLMNOPQRSTUVWXYZ" +

            "abcdefghijklmnopqrstuvwxyz";

​

    private static readonly int CharsAmount = Alphabet.Length;

​

    public static void Main(string[] args)

    {

        Tests();

    }

​

    static string GetBase62(Int64 base10)

    {

        bool isNegative = base10 < 0;

        decimal value = Math.Abs(base10);

        string base62 = GetChar(value).ToString();

        while (value >= CharsAmount)

        {

            value = Math.Floor(value / CharsAmount);

            base62 = GetChar(value) + base62;

        }

​

        if (isNegative)

        {

            base62 = "-" + base62;

        }

​

        return base62;

    }

​

    static char GetChar(decimal value)

    {

        int fraction = (int) (value % CharsAmount);

        return Alphabet[fraction];

    }

​

    static void Tests()

    {

        // <0

        TryTest(-10);

        // 0

        TryTest(0);

        // 0-61

        TryTest(15);

        // 62

        TryTest(62);

        // >62

        TryTest(10000);

        // >2,147,483,647

        TryTest(2147483648);

        // >2,147,483,647,000

        TryTest(2147483648000);

    }

​

    static void TryTest(Int64 value)

    {

        try

        {

            Console.WriteLine($"{value}(10) == {GetBase62(value)}({CharsAmount})");

        }

        catch (Exception e)

        {

            Console.WriteLine($"value:{value} error:{e.Message}");

        }

    }

}