using System;

using System.Text;

using System.Collections;

public class Program

{

    static char[] chars = new char[] {'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z'};

    static ArrayList strList = new ArrayList();

    public static void Main()

    {   

        int no = 1021;

        if(no <= 0) { Console.WriteLine( "Invalid input"); return; }

        int[] digits = new int[no.ToString().Length];

        int i = 0;

        while (no > 0)

        {

            int rem = no % 10;

            digits[i++] = rem;

            no = no / 10;

        }

        StringBuilder sb = new StringBuilder();

        Array.Reverse(digits);

        getPossibleStrings(digits, 0, sb);

        for(int j=0; j<strList.Count; j++) {

            Console.WriteLine(strList[j]);

        }

        Console.WriteLine("Output : "  + strList.Count);

    }

    public static void getPossibleStrings(int[] digits, int index, StringBuilder str)

    {

        if (index == digits.Length)

        {

            var listItem = str.ToString();

            if(!strList.Contains(listItem)) {

                strList.Add(listItem);

            }

            return;

        }

​

        if (digits[index] > 0)

        {

            var isNextDigitZero = index + 1 < digits.Length && digits[index+1] == 0 ? true : false;     

            if(!isNextDigitZero) {

                str.Append(chars[digits[index] - 1]);

                getPossibleStrings(digits, index + 1, str);

                str.Remove(str.Length - 1, 1);

            }

        } else {

            getPossibleStrings(digits, index + 1, str);

        }

​

        if (index + 1 < digits.Length)

        {

            int sumOfDigits = digits[index] * 10 + digits[index + 1];

            if (sumOfDigits > 0 && sumOfDigits <= 26)

            {

                str.Append(chars[sumOfDigits - 1]);

                getPossibleStrings(digits, index + 2, str);

                str.Remove(str.Length - 1, 1);

            } else

            {

                getPossibleStrings(digits, index + 2, str);

            }

        }

    }

}