Doubly Linked list
/*****************************************************************************
* Copyright (C) The Apache Software Foundation. All rights reserved. *
* ------------------------------------------------------------------------- *
* This software is published under the terms of the Apache Software License *
* version 1.1, a copy of which has been included with this distribution in *
* the LICENSE file. *
*****************************************************************************/
/**
* A simple Doubly Linked list class, designed to avoid
* O(n) behaviour on insert and delete.
*/
public class DoublyLinkedList {
/**
* Basic doubly linked list node interface.
*/
public static class Node {
private Node next = null;
private Node prev = null;
public final Node getNext() { return next; }
public final Node getPrev() { return prev; }
protected final void setNext(Node newNext) { next = newNext; }
protected final void setPrev(Node newPrev) { prev = newPrev; }
/**
* Unlink this node from it's current list...
*/
protected final void unlink() {
if (getNext() != null)
getNext().setPrev(getPrev());
if (getPrev() != null)
getPrev().setNext(getNext());
setNext(null);
setPrev(null);
}
/**
* Link this node in, infront of nde (unlinks it's self
* before hand if needed).
* @param nde the node to link in before.
*/
protected final void insertBefore(Node nde) {
// Already here...
if (this == nde) return;
if (getPrev() != null)
unlink();
// Actually insert this node...
if (nde == null) {
// empty lst...
setNext(this);
setPrev(this);
} else {
setNext(nde);
setPrev(nde.getPrev());
nde.setPrev(this);
if (getPrev() != null)
getPrev().setNext(this);
}
}
}
private Node head = null;
private int size = 0;
public DoublyLinkedList() {}
/**
* Returns the number of elements currently in the list.
*/
public synchronized int getSize() { return size; }
/**
* Removes all elements from the list.
*/
public synchronized void empty() {
while(size > 0) pop();
}
/**
* Get the current head element
* @return The current 'first' element in list.
*/
public Node getHead() { return head; }
/**
* Get the current tail element
* @return The current 'last' element in list.
*/
public Node getTail() { return head.getPrev(); }
/**
* Moves <tt>nde</tt> to the head of the list (equivilent to
* remove(nde); add(nde); but faster.
*/
public void touch(Node nde) {
if (nde == null) return;
nde.insertBefore(head);
head = nde;
}
/**
* Adds <tt>nde</tt> to the head of the list.
* In perl this is called an 'unpop'. <tt>nde</tt> should
* not currently be part of any list.
* @param nde the node to add to the list.
*/
public void add(Node nde) {
if (nde == null) return;
nde.insertBefore(head);
head = nde;
size++;
}
/**
* Removes nde from the list it is part of (should be this
* one, otherwise results are undefined). If nde is the
* current head element, then the next element becomes head,
* if there are no more elements the list becomes empty.
* @param nde node to remove.
*/
public void remove(Node nde) {
if (nde == null) return;
if (nde == head) {
if (head.getNext() == head)
head = null; // Last node...
else
head = head.getNext();
}
nde.unlink();
size--;
}
/**
* Removes 'head' from list and returns it. Returns null if list is empty.
* @returns current head element, next element becomes head.
*/
public Node pop() {
if (head == null) return null;
Node nde = head;
remove(nde);
return nde;
}
/**
* Removes 'tail' from list and returns it. Returns null if list is empty.
* @returns current tail element.
*/
public Node unpush() {
if (head == null) return null;
Node nde = getTail();
remove(nde);
return nde;
}
/**
* Adds <tt>nde</tt> to tail of list
*/
public void push(Node nde) {
nde.insertBefore(head);
if (head == null) head = nde;
size++;
}
/**
* Adds <tt>nde</tt> to head of list
*/
public void unpop(Node nde) {
nde.insertBefore(head);
head = nde;
size++;
}
}
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