def merge_two_sorted_lists(self, l1: ListNode, l2: ListNode) -> ListNode:
    if (not l1) or (l2 and l1.val > l2.val):
        l1, l2 = l2, l1
    if l1:
        l1.next = self.merge_two_sorted_lists(l1.next, l2)
    return l1
    
