双向链表：

就是有双向指针 即 双向的链域

链结点的结构：

┌────┬────┬────────┐

│data│next│previous│

└────┴────┴────────┘

双向链表不必是双端链表(持有对最后一个链结点的引用)，双端链表插入时是双向的

有两条链：一条从头到尾，一条从尾到头，删除遍历时也是双向的

/**
 * 双向链表
 * 
 * @author stone
 */
public class DoublyLinkedList<T> {
	private Link<T> head;		//首结点
	private Link<T> rear;		//尾部指针
	
	public DoublyLinkedList() {
		
	}
	
	public T peekHead() {
		if (head != null) {
			return head.data;
		}
		return null;
	}
	
	public boolean isEmpty() {
		return head == null;
	}
	
	public void insertFirst(T data) {// 插入 到 链头
		Link<T> newLink = new Link<T>(data);
		if (isEmpty()) {//为空时，第1次插入的新结点为尾结点
			rear = newLink;
		} else {
			head.previous = newLink; //旧头结点的上结点等于新结点
		}
		newLink.next = head; //新结点的下结点旧头结点
		head = newLink; //赋值后，头结点的下结点是旧头结点，上结点null
	}
	
	public void insertLast(T data) {//在链尾 插入
		Link<T> newLink = new Link<T>(data);
		if (isEmpty()) {
			head = newLink;
		} else {
			rear.next = newLink;
		}
		newLink.previous = rear;
		rear = newLink; //赋值后，尾结点的上结点是旧尾结点，下结点null
	}
	
	public T  deleteHead() {//删除 链头
		if (isEmpty()) return null;
		Link<T> temp = head;
		head = head.next; //变更首结点，为下一结点
		if (head != null) {
			head.previous = null;
		} else {
			rear = null;
		}
		return temp.data;
	}
	
	public T  deleteRear() {//删除 链尾
		if (isEmpty()) return null;
		Link<T> temp = rear;
		rear = rear.previous; //变更尾结点，为上一结点
		if (rear != null) {
			rear.next = null;
		} else {
			head = null;
		}
		return temp.data;
	}
	
	public T find(T t) {//从头到尾find
		if (isEmpty()) {
			return null;
		}
		Link<T> find = head;
		while (find != null) {
			if (!find.data.equals(t)) {
				find = find.next;
			} else {
				break;
			}
		}
		if (find == null) {
			return null;
		}
		return find.data;
 	}
	
	public T delete(T t) {
		if (isEmpty()) {
			return null;
		}
		Link<T> current = head;
		while (!current.data.equals(t)) {
			current = current.next;
			if (current == null) {
				return null;
			}
		}
		if (current == head) {
			head = head.next;
			if (head != null) {
				head.previous = null;
			}
		} else if (current == rear) {
			rear = rear.previous;
			if (rear != null) {
				rear.next = null;
			}
		} else {
			//中间的非两端的结点，要移除current
			current.next.previous = current.previous;
			current.previous.next = current.next;
		}
		return current.data;
	}
	
	public boolean insertAfter(T key, T data) {//插入在key之后, key不存在return false
		if (isEmpty()) {
			return false;
		}
		Link<T> current = head;
		while (!current.data.equals(key)) {
			current = current.next;
			if (current == null) {
				return false;
			}
		}
		Link<T> newLink = new Link<T>(data);
/*		if (current == head) {//若为头，考虑头的后指针，原头后指针的前指针，新链结点的前后指针
			newLink.next = current.next;
			current.next.previous = newLink;
//			newLink.previous = current;
//			current.next = newLink;
		} else if (current == rear) {
//			newLink.next = null;
			rear = newLink;
//			current.next = newLink;
//			newLink.previous = current;
		} else {
			newLink.next = current.next;
			current.next.previous = newLink;
//			current.next = newLink;
//			newLink.previous = current;
		}*/
		//上面的判断简写成：
		if (current == rear) {
			rear = newLink;
		} else {
			newLink.next = current.next;
			current.next.previous = newLink;
		}
		current.next = newLink;
		newLink.previous = current;
		return true;
	}
	
	public void displayList4Head() {//从头开始遍历
		System.out.println("List (first-->last):");
		Link<T> current = head;
		while (current != null) {
			current.displayLink();
			current = current.next;
		}
	}
	public void displayList4Rear() {//从尾开始遍历
		System.out.println("List (last-->first):");
		Link<T> current = rear;
		while (current != null) {
			current.displayLink();
			current = current.previous;
		}
	}
	
	class Link<T> {//链结点
		T data;		//数据域
		Link<T> next; //后继指针，结点			链域
		Link<T> previous; //前驱指针，结点		链域
		Link(T data) {
			this.data = http://www.mamicode.com/data;>
打印
List (first-->last):
the data is 4
the data is 2
the data is 1
the data is 3
the data is 5
deleteHead:4
List (first-->last):
the data is 2
the data is 1
the data is 3
the data is 5
deleteRear:5
List (last-->first):
the data is 3
the data is 1
the data is 2
find:null
find:3
delete find:null
delete find:1
List (first-->last):
the data is 2
the data is 3
----在指定key后插入----
List (first-->last):
the data is 2
the data is 9
the data is 10
the data is 8
the data is 3


Java 实现双向链表