using System;

using System.Collections.Generic;

using System.Linq;

using System.Text.RegularExpressions;

using System.Diagnostics;

public class Program

{

    public static void Main(string[] args)

        {

            //Your code goes here

            String str;

            var stopwatch = new Stopwatch();

            stopwatch.Start();

            str = AppendFunc("", ", ", "hello", "world", "", "Test");

            stopwatch.Stop();

            Console.WriteLine("linq: " + str + " - "+ stopwatch.ElapsedTicks);

            stopwatch.Reset();

            str = "";

            var stopwatch2 = new Stopwatch();

            stopwatch2.Start();

            str = AppendFunc2("", ", ", "hello", "world", "", "Test");

            stopwatch2.Stop();

            Console.WriteLine("conventional: " + str + " - "+ stopwatch2.ElapsedTicks);

        }

        public static string AppendFunc(string str, string separator, params string[] values) {

            str = str ?? string.Empty;

            if (!string.IsNullOrEmpty(str)) {

                str += ", " + String.Join(separator, values.Where(s => !string.IsNullOrEmpty(s)));

            } else {

                str = String.Join(separator, values.Where(s => !string.IsNullOrEmpty(s)));

            }

            return str;

        }

        public static string AppendFunc2(string str, string separator, params string[] values) {

            str = str ?? string.Empty;

            foreach (string value in values)

            {

                if (!string.IsNullOrEmpty(value))

                {

                    str += separator;

                    str += value;

                }

            }

            return str;

        }

}