using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

​

public class Program

{

    public static void Main()

    {

        for(;;)

        {

            var value = ReadInteger("Please provide a number: ");

            var digits = new Stack<int>(GetDigits(value).Reverse());

​

            foreach (var combinaison in GetCombinations(digits))

            {

                Console.WriteLine(GetString(combinaison));

            }

            Console.WriteLine();

        }

    }

​

    static IEnumerable<IReadOnlyList<int>> GetCombinations(Stack<int> stack)

    {

        return GetCombinationsImplem(stack, new Stack<int>());

    }

​

    static IEnumerable<IReadOnlyList<int>> GetCombinationsImplem(Stack<int> inStack, Stack<int> outStack)

    {

        // recursion end point

        if (inStack.Count == 0)

        {

            yield return outStack.ToArray();

            yield break;

        }

​

        var digit1 = inStack.Pop();

​

        // Try put one digit from in to out

        if (digit1 >= 1 && digit1 <= 26)

        {

            outStack.Push(digit1);

            foreach (var combinaison in GetCombinationsImplem(inStack, outStack))

            {

                yield return combinaison;

            }

​

            outStack.Pop();

        }

​

        // Try put two combined digit (1,2 => 12) from in to out

        if (inStack.Count > 0)

        {

            var digit2 = inStack.Pop();

            var v = digit1 + 10 * digit2;

            if (v >= 1 && v <= 26)

            {

                outStack.Push(v);

                foreach (var combinaison in GetCombinationsImplem(inStack, outStack))

                {

                    yield return combinaison;

                }

                outStack.Pop();

            }

            inStack.Push(digit2);

        }

​

        inStack.Push(digit1);

    }

​

    static IEnumerable<int> GetDigits(int value)

    {

        while (value > 0)

        {

            yield return value % 10;

            value /= 10;

        }

    }

​

    static string GetString(IReadOnlyCollection<int> digits)

    {

        var sb = new StringBuilder(digits.Count);

        sb.Append(string.Join("|", digits));

        sb.Append(" => ");

        foreach (var digit in digits)

        {

            // Convert 1 to 'a', 2 to 'b', ...

            sb.Append((char)('a' + digit - 1));

        }

​

        return sb.ToString();

    }

​

    static int ReadInteger(string caption)

    {

        Console.Write(caption);

        for (;;)

        {

            int value;

            var line = Console.ReadLine();

            if (line != null && int.TryParse(line, out value))

                return value;

​

            Console.WriteLine("This was not a number.");

        }

    }

}