using System;

using System.Collections;

using System.Collections.Generic;

public class Program

{

    private static Dictionary<int, string> dictNumbers = new Dictionary<int, string>() 

    {

        {0, ""}, {1, ""}, {2, "abc"}, {3, "def"}, {4, "ghi"}, {5, "jkl"}, {6, "mno"}, {7, "pqrs"}, {8, "tuv"}, {9, "wxyz"}

    };

    public static void Main()

    {

        //Console.WriteLine(dictNumbers[5]);

        NumToWord(new int[]{2, 4});

    }

    private static void NumToWord(int[] nums)

    {

        printWordsUtil(nums, 0, new char[nums.Length], nums.Length);

    }

    private static void printWordsUtil(int[] number, int currentDigit, char[] output, int n)

    {

        if(currentDigit == n)

        {

            string s = new string(output);

            Console.WriteLine(s);

            return;

        }

        for (int i=0; i<dictNumbers[number[currentDigit]].Length; i++)

        {

            output[currentDigit] = dictNumbers[number[currentDigit]].ToCharArray()[i];

            printWordsUtil(number, currentDigit+1, output, n);

            if (number[currentDigit] == 0 || number[currentDigit] == 1)

                return;

        }

    }

}

​

​