Обединение и сечение

using System;
using System.Collections.Generic;

namespace Demo
{
    class Program
    {
        static void Main(string[] args)
        {
            List<int> firstList = new List<int> { 3, 4, 5, 6 };
            List<int> secondsList = new List<int> { 1, 2, 4, 5, 7 };
            List<int> union = Union(firstList, secondsList);
            List<int> intersect = Intersect(firstList, secondsList);





        }

        public static List<int> Union( List<int> firstList, List<int> secondList)
        {
            List<int> union = new List<int>();
            union.AddRange(firstList);
            foreach (var item in secondList)
            {
                if (!union.Contains(item))
                {
                    union.Add(item);
                }
            }
            return union;
        }

        public static List<int> Intersect(List<int> firstList,  List<int> secondList)
        {
            List<int> intersect = new List<int>();
            foreach (var item in firstList)
            {
                if (secondList.Contains(item))
                {
                    intersect.Add(item);
                }
            }
            return intersect;
        }


    }
}