Merge Two Sorted Lists

using System;
using System.Collections.Generic;

namespace leetcodecsharp
{
    class Program
    {
        public class ListNode
        {
            public int val;
            public ListNode next;
            public ListNode(int val = 0, ListNode next = null)
            {
                this.val = val;
                this.next = next;
            }
        }
        public static ListNode Function(ListNode list1, ListNode list2)
        {
            ListNode c = null;
            ListNode temp = null;
            if (list1 == null && list2 == null)
                return null;
            if (list1 == null)
                return list2;
            if (list2 == null)
                return list1;
            while (list1 != null && list2 != null)
            {
                if (list1.val < list2.val)
                {
                    c = insert(c, list1.val);
                    list1 = list1.next;
                }
                else
                {
                    c = insert(c, list2.val);
                    list2 = list2.next;
                }
            }
            if (list1 == null)
            {
                temp = c;
                while (temp.next != null)
                {
                    temp = temp.next;
                }
                temp.next = list2;
            }
                
            if (list2 == null)
            {
                temp = c;
                while (temp.next != null)
                {
                    temp = temp.next;
                }
                temp.next = list1;
            }
            return c;
        }

        public static ListNode insert(ListNode root, int item)
        {
            ListNode temp = new ListNode();
            ListNode ptr;
            temp.val = item;
            temp.next = null;

            if (root == null)
                root = temp;
            else
            {
                ptr = root;
                while (ptr.next != null)
                    ptr = ptr.next;
                ptr.next = temp;
            }
            return root;
        }
        
        public static ListNode ArrayToList(int []arr, int n)
        {
            ListNode root = null;
            for (int i = 0; i < n; i++)
            {
                root = insert(root, arr[i]);
            }
            return root;
        }


        public static void Main(string[] args)
        {
            int []list1 = { 5 };
            int []list2 = { 1, 2, 4 };
            int n1 = 1;
            int n2 = 3;
            ListNode a = ArrayToList(list1, n1);
            ListNode b = ArrayToList(list2, n2);
            //ListNode b = null;
            Console.WriteLine(Function(a, b));
        }

    }
}