using System;

public class Program

{

    public static void Main()

    {

        int [] arr1 = new int[] { 1, 2, 3, 4 };

        int[] arr2 = new int[] { 3, 4, 5, 6, 7, 8 };

        int[][] complementArr = getComplement(arr1, arr2);

        int[][] differenceArr = getComplement(arr2, complementArr[0]);

        int[] arr3 = differenceArr[1];

        int[] arr4 = complementArr[1];

        Console.WriteLine(arr3[0]);

        Console.WriteLine(arr4[0]);

    }

    public static int[][] getComplement(int[] arr1, int[] arr2) {

        int[] complement = {};

        int[] difference = {};

​

        for (int i = 0; i < arr1.Length; i++)

        {

            bool isDupe = false;

            for (int j = 0; j < arr2.Length; j++) {

                if (arr1[i] == arr2[j] && !isDupe) {

                    Array.Resize(ref complement, complement.Length + 1);

                    complement[complement.GetUpperBound(0)] = arr2[j];

                    isDupe = true;

                }

            }

            if (!isDupe) {

                Array.Resize(ref difference, difference.Length + 1);

                difference[difference.GetUpperBound(0)] = arr1[i];

            }

        }

        return new[] { complement, difference };

    }

}