using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Initiating Test");

        Console.WriteLine(Environment.NewLine);

        Console.WriteLine(Environment.NewLine);

​

        var testList = new List<List<string>>();

        testList.Add(usernames(new List<string> { "ken", "paul" , "ken", "hani", "ken" }));

        testList.Add(usernames(new List<string> { "ken", "paul"}));

        testList.Add(usernames(new List<string> { "ken", "paul" , "ken", "hani", "ken", "ken" }));

        testList.Add(usernames(new List<string> { "steve", "steve" , "paul", "steve", "matt" }));

        testList.Add(usernames(new List<string> { "matt", "paul" }));

        testList.Add(usernames(new List<string> { "paul", "hani" , "hani", "hani", "paul" }));

        testList.Add(usernames(new List<string> { "tatum", "charles" , "ken", "paul", "ken", "tatum", "tatum", "tatum", "paul", "paul" }));

​

​

        foreach (var test in testList){

            foreach (var testItem in test){

                Console.WriteLine(testItem);

            }

            Console.WriteLine(Environment.NewLine);

        }

    }

    public static List<string> usernames(List<string> u){

        HashSet<string> hashSet = new HashSet<string>();

        for ( var i = 0; i < u.Count; i++){

            if (hashSet.Contains(u[i])){

                var dupeCount = hashSet.Where(a=> a.Contains(u[i])).Count();

                hashSet.Add(MakeUnique(u[i], dupeCount));

            }

            else{

                hashSet.Add(u[i]);

            }       

        }

        return new List<string>(hashSet);

    }

    public static string MakeUnique(string value, int count){

        return string.Format("{0}{1}", value, count);       

    }

}