External Merge Sort

class Program
    {

        //External Merge Sort
        static int[] F = { 75, 55, 15, 20, 85, 30, 35, 10, 60, 40, 50, 25, 45, 80, 70, 65 };
        //static int[] F = { 0, 55, 15, 20, 85, 30, 35, 10, 9, 40, 125, 45, 80, 6, 65 };

        static int[] F1 = new int[F.Length];
        static int[] F2 = new int[F.Length];
        static void Main(string[] args)
        {
            Console.Write("F = ");
            foreach(var item in F)
            {
                Console.Write(item+" ");
            }

            F1[0] = F[0];
            split(F1, F2, 1, 0, F, 1);
            Console.WriteLine("----------------------Sorted-------------------");
            foreach(var item in F)
            {
                Console.WriteLine(item+" ");
            }
            Console.WriteLine("\n"+ "------------------------------------------");
        }
        static void split(int []F1,int[] F2,int F1_index,int F2_index, int[] F ,int F_index)
        {
            if(F_index < F.Length)
            {
                if(F1[F1_index - 1] < F[F_index])
                {
                    F1[F1_index] = F[F_index];
                    split(F1, F2, F1_index + 1, F2_index, F, F_index + 1);
                }
                else
                {
                    F2[F2_index] = F[F_index];
                    split(F2, F1, F2_index + 1, F1_index, F, F_index + 1);

                }
            }
            else
            {
                Console.WriteLine("\n"+"------------------Split--------------");
                Console.WriteLine("F1 = ");
                foreach(var item in F1)
                {
                    Console.Write(item + " ");
                }
                Console.Write("\n" + "F2 = ");
                foreach(var item in F2)
                {
                    Console.WriteLine(item + " ");
                }
                Console.WriteLine("\n"+"---------------------------------------");
                merge(F1_index, F2_index, F1, F2);
            }
        }
        static void merge(int F1_index,int F2_index,int [] F1,int [] F2)
        {
            if(F2_index != 0)
            {
                int j = 0;
                int k = 0;
                for (int i = 0; i < F.Length; i++)
                {
                    if(F1[j] < F2[k])
                    {
                        if(j < F1_index)
                        {
                            F[i] = F1[j];
                            j = j + 1;
                        }
                        else
                        {
                            F[i] = F2[k];
                            k = k + 1;
                        }
                    }
                    else
                    {
                        if (k < F2_index)
                        {
                            F[i] = F2[k];
                            k = k + 1;
                        }
                        else
                        {
                            F[i] = F1[j];
                            j = j + 1;
                        }
                    }
                }
                for (int i = 0; i < F1.Length; i++)
                {
                    F1[i] = 0;
                    F2[i] = 0;
                }

                Console.WriteLine("\n" + "----------------------------------Merge------------");
                Console.Write("F = ");
                foreach(var item in F)
                {
                    Console.Write(item + " ");
                }

                Console.WriteLine("\n"+"---------------------------------------------------------");
                F1[0] = F[0];
                split(F1, F2, 1, 0, F, 1);
            }
        }
    }