using System;

public class Program

{

    public static void Main()

    {

        var rr= Increment("","ALPHA").ToUpper(); //Retunr A

        Console.WriteLine(rr);  

        var rr1 = Increment("a","ALPHA").ToUpper(); //Retunr D

        Console.WriteLine(rr1); 

        var rr2 = Increment("v","ALPHA").ToUpper(); //Retunr W

        Console.WriteLine(rr2); 

        var rr3 = Increment("z","ALPHA").ToUpper(); //Retunr AA

        Console.WriteLine(rr3); 

        var rr4 = Increment("ab","ALPHA").ToUpper(); //Retunr AC

        Console.WriteLine(rr4); 

        var rr5= Increment("","NonAlpha").ToUpper(); //Retunr A

        Console.WriteLine(rr5); 

        var rr6 = Increment("5","NonAlpha").ToUpper(); //Retunr D

        Console.WriteLine(rr6); 

        var rr7 = Increment("10","NonAlpha").ToUpper(); //Retunr D

        Console.WriteLine(rr7); 

        var rr8 = Increment("150","NonAlpha").ToUpper(); //Retunr D

        Console.WriteLine(rr8); 

    }   

    static string Increment(string s, string type)

    {

        if(type != "ALPHA")

            {

                if ((s == null) || (s.Length == 0))

                return "1";

                int number= Int32.Parse(s);

                if ( number >= 0 )

                {

                    number= number + 1;

                }

                return number.ToString();

            }

        else {

            return  AlphaIncrement(s);

        }

    }

    static string AlphaIncrement(string s)

        {       

            // first case - string is empty: return "a"

            if ((s == null) || (s.Length == 0))

                return "a";

​

            // next case - last char is less than 'z': simply increment last char

            char lastChar = s[s.Length - 1];

            string fragment = s.Substring(0, s.Length - 1);

​

            if (lastChar < 'z')

            {

                ++lastChar;

                return fragment + lastChar;

            }

​

            // next case - last char is 'z': roll over and increment preceding string

            return AlphaIncrement(fragment) + 'a';

        }

}