using System.Collections.Generic;

using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");       

        // union of two list        

        var list1 = new List<string>{ "a", "b", "c" };

        var list2 = new List<string>{ "b", "d", "c" };      

        var result = Union(list1, list2);       

        foreach(var item in result)

        {

            Console.WriteLine(item);

        }

    }

    public static List<T> Union<T>(List<T> list1, List<T> list2)

    {

        var result = new HashSet<T>();

        foreach(var item in list1)

        {

            result.Add(item);

        }

        foreach(var item in list2)

        {

            if(!result.Contains(item))

            {

                result.Add(item);

            }

        }

        return result.ToList(); 

    }

}