using System;

using System.Collections.Generic;

public 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 class Solution {

    public ListNode MergeTwoLists(ListNode l1, ListNode l2) {

        if (l1 == null && l2 == null) return null;

        else if (l1 == null) return l2;

        else if (l2 == null) return l1;

        else if (l1.val < l2.val) {

            l1.next = MergeTwoLists(l1.next, l2);

            return l1;

        }

        else {

            l2.next = MergeTwoLists(l1, l2.next);

            return l2;

        }

    }

    public static void Main()

    {

​

    }

}