Java - 删除整个链表不会影响实际列表

Question

I typed recursive method to delete all the nodes in a simple linked-list but in fact debugger shows this method doesn't affect the actual list and I don't know why it's like that. Here's my code:

class List
{ Element root; ...}

class Element
{int value; Element next; ...}


Element delete(Element head)
{
 if(head == null) return null;
 head = delete(head.next);
 return head;
}

Answer 1

There's nothing in your delete method that actually deletes anything, ie it doesn't change the list and it doesn't change any Element in the list. All it does is search through the entire list to find an Element whose .next() is null, then it returns null.

You have to actually change a value somewhere.

So let's start with the delete(Element) method. As I've come to understand, it needs to take an Element, which is the head of a List, and delete all members of the List. Setting head = null won't work, as @Dan Getz has noted. You need to be able to get at the actual list, and all you have to work with is the Element, so the Element has to know which List contains it. And that means you need a myList field of type List in Element. Then your delete(Element head) method could just do

head.myList.deleteAll();

That requires a deleteAll() method in List, which would set root = null;

Or, if root is accessible from your delete(Element head) method, you could do

head.myList.root = null;

The point is that you somehow have to get to root, and passing it in to some other method doesn't do the job.

Old answer below, doesn't quite fit the assignment requirements

Start by removing the argument. You don't need to pass in head.

Then, in the body, just set head = null (assuming you have a field called head).

Return either null or the previous value of head, whichever matches your API.