using System;

using NUnit.Framework;

​

public class Program

{

    static readonly string letters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

​

    public static void Main()

    {

        var result = "";

        result = Program.GetNextColumn("A");

        Assert.AreEqual("B", result);

        result = Program.GetNextColumn("Z");

        Assert.AreEqual("AA", result);

​

        result = Program.GetNextColumn("AZ");

        Assert.AreEqual("BA", result);

​

        result = Program.GetNextColumn("ABZ");

        Assert.AreEqual("ACA", result);

​

        result = Program.GetNextColumn("ZZZZ");

        Assert.AreEqual("AAAAA", result);

        result = Program.GetNextColumn("WSZ");

        Assert.AreEqual("WTA", result);

​

    }

    private static string GetNextColumn(string str){

        char lastChar = Convert.ToChar(str.Substring(str.Length - 1, 1));

        string retVal = "";

        for(int i = str.Length - 1; i >= 0; i--){

            int currIndex = letters.IndexOf(str.Substring(i, 1));

            if(currIndex < 25){ // not Z

                if(i == str.Length - 1){ //Last Letter

                    retVal = letters.Substring(currIndex + 1, 1) + retVal;

                }

                else if(i < str.Length - 1 && i > 0){ //Mid Letter

                    if(str.Substring(i + 1, 1) == "Z")

                        retVal = letters.Substring(currIndex + 1, 1) + retVal;

                    else

                        retVal = letters.Substring(currIndex, 1) + retVal;

                }

                else

                {

                    if(retVal.Substring(0, 1) != "Z" && retVal.Substring(0, 1) != "A")

                        retVal = letters.Substring(currIndex, 1) + retVal;

                    else

                        retVal = letters.Substring(currIndex + 1, 1) + retVal;

                }

            }

            else // is Z ABZ

            {

                if(i == str.Length - 1 && i > 0){ //Last Letter

                        retVal = letters.Substring(0, 1) + retVal;

                }

                else if(i < str.Length - 1 && i > 0)

                {

                    if(str.Substring(i - 1, 1) != "A" && str.Substring(i - 1 , 1) != "Z")

                        retVal = letters.Substring(currIndex, 1) + retVal;

                    else

                        retVal = letters.Substring(0, 1) + retVal;

                }

                else

                {

                    retVal = letters.Substring(0, 1) + retVal;

                    if(i == 0){

                        retVal = letters.Substring(0, 1) + retVal;

                    }

                }

            }

        }

        return retVal;

    }

}