21. Merge Two Sorted Lists

  public ListNode MergeTwoLists(ListNode l1, ListNode l2) {
        if(l1 == null) return l2;
        if(l2 == null) return l1;
        if(l1.val<l2.val) 
        {
            l1.next =  MergeTwoLists(l1.next,l2);
            return l1;
        }
        else
        {
            l2.next = MergeTwoLists(l1,l2.next);
            return l2;
        }
    }

public ListNode MergeTwoLists(ListNode l1, ListNode l2) {
       if (l1 == null) return l2;
            if (l2 == null) return l1;
           
            var sentinel = new ListNode(-1);
            var dummy = sentinel;
            while(l1 != null && l2 != null)
            {
                if (l1.val < l2.val)
                {
                    sentinel.next = l1;
                    l1 = l1.next;
                }
                else
                {
                    sentinel.next = l2;
                    l2 = l2.next;
                }
                sentinel = sentinel.next;
            }
            if (l1 != null) sentinel.next = l1;
            else sentinel.next = l2;
            return dummy.next;
    }