（十二）链表—双向链表

一、双向链表说明：

1. 以前所遇到的链表都只提供了从前向后遍历数组的功能，而双向链表提供了从最后向前遍历数组的功能。这是因为在双向链表中都有两个指向其他链表的引用。

2. 当然，双向链表也有它的缺点：每次插入和删除链结点的时候，都要处理四个引用；此外，双向链表的链结点比其他链表的链结点占用的空间要多。双向链表如下图所示：

二、Java语言描述双向链表：

package com.solid.link;

public class DoublyLinkedList {

//对第一个链结点的引用

private Link first;

//对最后一个链结点的引用

private Link last;

/**

* 构造方法

*/

public DoublyLinkedList() {

first = null;

last = null;

}

/**

* 判断是否为空

* @return

*/

public boolean isEmpty() {

return (first==null);

}

/**

* 插入到链表首部

* @param key

*/

public void insertFirst(int key) {

Link link = new Link(key);

if(isEmpty()) {

last = link;

} else {

first.previous = link;

}

link.next = first;

first = link;

}

/**

* 插入到链表最后

* @param key

*/

public void insertLast(int key) {

Link link = new Link(key);

if(isEmpty()) {

first = link;

} else {

last.next = link;

link.previous = last;

}

last = link;

}

/**

* 删除第一个链结点

*/

public void deleteFirst() {

if(first.next == null) {

last = null;

}

first.next.previous = null;

first = first.next;

}

/**

* 删除最后一个链结点

*/

public void deleteLast() {

if(first.next == null) {

first = null;

}

last.previous.next = null;

last = last.previous;

}

/**

* 删除任意一个链结点

* @param key

*/

public Link delete(int key) {

Link current = first;

while(current.iDate != key) {

current = current.next;

if(current == null) {

return null;

}

}

if(current == first)

first = current.next;

else

current.previous.next = current.next;

if(current == last)

last = current.previous;

else

current.next.previous = current.previous;

return current;

}

/**

* 从前向后遍历

*/

public void displayForward() {

Link current = first;

System.out.print("first-->last: ");

while(current != null) {

current.display();

current = current.next;

}

System.out.println();

}

/**

* 从后向前遍历

*/

public void displayBackward() {

Link current = last;

System.out.print("last-->first: ");

while(current != null) {

current.display();

current = current.previous;

}

System.out.println();

}

}