public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
      ListNode dummy = new ListNode(0);
      ListNode cur = dummy;

      while (l1 != null && l2 != null) {
          if (l1.val < l2.val) {
              cur.next = l1;
              l1 = l1.next;
          }
          else {
              cur.next = l2;
              l2 = l2.next;
          }
          cur = cur.next;
      }
      if(l1 != null && l2 == null) {
          cur.next = l1;
      }
      if (l1 == null && l2 != null)
      cur.next = l2;

    return dummy.next;
    }