双向链表的 Remove() 方法

Question

 /**
 * The remove() method removes the current node from the list. If the next node exists,
 * it becomes the current node. Otherwise the previous node becomes the current node. 
 * If neither node exists, the list becomes empty.
 * 
 * @throws Exception    If the list is empty.
 */

public void remove() throws Exception
{
    if (isEmpty())
    {
        throw exListEmpty;
    }
    Node target = current;      
    Node prevNode = target.prev;
    Node nextNode = target.next;

    // current node is at head
    if (prevNode == null)
    {
        nextNode.prev = null;
        head = nextNode;
    }
    else
    {
        prevNode.next = nextNode;
        current = prevNode;
    }

    // current node is at tail
    if (nextNode == null)
    {
        prevNode.next = null;
        tail = prevNode;
    }
    else
    {
        nextNode.prev = prevNode;
        current = nextNode;
    }
    if (prevNode == null && nextNode == null)
    {
        isEmpty();
    }
    else if (prevNode != null && nextNode != null)
    {
        prevNode.next = nextNode;
        nextNode.prev = prevNode;
        current = nextNode;
    }
    size--; 
}

The test code is:

    public void testContains() throws Exception
    {
        PDLinkedList first = new PDLinkedList();
        first.append(5);
        first.insert(7);
        first.add(8);

        System.out.println("Before first remove: " + first.toString());
        first.remove(); 
        System.out.println("After first remove: " + first.toString());
        first.remove(5);
        System.out.println("After second remove: " + first.toString());
        first.remove();
        System.out.println("After third remove: " + first.toString());
    }

Which then prints out as:

Before first remove: 7 8 5
After first remove: 7 8 5
After second remove: 7 8
After third remove: 7 8

It should remove the 8 after the first remove(), then the 7 after the third remove(). My add(item) method is set as the current pointer, and my remove() method should then remove the node at the current pointer. But it seems to do nothing. I wonder if it is pointing at the wrong node? The code compiles, but I receive an Assertion Error that states the 7 and 8 are still in the list.

Answer 1

Consider removing a node in 5 distinct cases:

1. List is empty
2. List contains only one element
3. Target node is in the beginning
4. Target node is in the end
5. Target node is at other position

Do no mash everything in one long function, break it into corresponding steps:

(1) Is easy, just return

(2) You make both head and last point to null

(3) You assign head to head.next , and head.previous to null ( head is already "new" head)

void RemoveAtStart()
{
    if (IsEmpty())
    {
        return;
    }

    if (Head == Last)
    {
        Head = null;
        Last = null;
    }
    else
    {
        Head = Head.Next;
        Head.Previous = null;
    }       
}

(4) Is very similar: last becomes last.previous , and last.next to null ( last is "new" last)

void RemoveAtEnd()
{
    if (IsEmpty())
    {
        return;
    }

    if (Last == Head)
    {
        Last = null;
        Head = null;            
    }
    else
    {
        Last = Last.Previous;
        Last.Next = null;
    }       
}

(5) You find target , then point target.previous.next to target.next ("left" node of target skips it, and points to the right node of target). Then, if target is not the last, point target.Next.Previous to target.Previous .

Then you combine all cases 1-5 :

void Remove(Node<T> node)
{
    if (IsEmpty())
    {
        return;
    }

    if (Head == Last)
    {
        Head = null;
        Last = null;
    }
    else if (node == Last)
    {
        RemoveAtEnd();
    }
    else if (node == Head)
    {
        RemoveAtStart();
    }
    else
    {
        Node<T> current = Head;
        while (current != node)
        {
            current = current.Next;
        }
        current.Previous.Next = current.Next;
        if (current.Next != null)
        {
            current.Next.Previous = current.Previous;
        }        
        current = null;        
    }       
}