单链表只能从前往后遍历，如果链表的长度较大，遍历到链表后半部分的时候想要往前查找，就只能回到开头，重新遍历了。
双向链表提供了这个能力，即允许前向遍历，也允许后向遍历整个链表。原因是双向链表的每个节点都有两个指向其他节点的引用。但这也是其缺点，因为在插入、删除的时候需要处理四个链接点的引用， 占用的空间也大了一些。如将头节点和尾节点链接起来，即成为双向循环链表。
下面是java代码：
package test;
public class DoubleLink {
public Link first;
public Link last;
public DoubleLink() {// 构造器，初始化
this.first = null;
this.last = null;
}
public boolean isEmpty() {// 判断是否为空
return first == null;
}
public void insertFirst(int idata) {// 将元素插入链表开头
Link link = new Link(idata);
if (isEmpty())
last = link;// 如果为空，last需要改变
else
first.previous = link;// 非空，则要在first前插入
link.next = first;
first = link;
}
public void insertLast(int idata) {// 插入链表结尾
Link link = new Link(idata);
if (isEmpty())
first = link;
else
last.next = link;
link.previous = last;
last = link;
}
public boolean insertAfter(int key, int idata) {// 在某项元素后插入
Link current = first;
while (current.idata != key) {//从头开始查找
current = current.next;
if (current == null)//到表尾也没有找到
return false;
}
Link link = new Link(idata);
if (current == last) {
link.next = null;
last = link;
} else {
link.next = current.next;
current.next.previous = link;
}
link.previous = current;
current.next = link;
return true;
}
public Link delectKey(int key) {// 删除某项元素
Link current = first;
while (current.idata != 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;
}
public Link delectFirst() {// 删除链表开头元素
Link temp = first;
if (first.next == null)// 只有一个元素
last = null;
else
first.next.previous = null;//first节点的next字段引用的链节点的previous字段
first = first.next;
return temp;
}
public Link delectLast() {// 删除链表最后的元素
Link temp = last;
if (first.next == null)
first = null;
else
last.previous.next = null;
last = last.previous;
return temp;
}
public void showFirst() {// 前向展示
Link current = last;
while (current != null) {
current.showLink();
current = current.previous;
}
}
public void showLast() {// 后向展示
Link current = first;
while (current != null) {
current.showLink();
current = current.next;
}
}
public static void main(String[] args) {
DoubleLink dlink = new DoubleLink();
dlink.insertFirst(1);
dlink.insertFirst(2);
dlink.insertFirst(3);
dlink.showFirst();
dlink.insertLast(4);
dlink.insertLast(5);
dlink.showFirst();
}
}
class Link {
public int idata;// 存放的数据
public Link previous;// 对前一项的引用
public Link next;// 对后一项的引用
public Link(int idata) {
this.idata = idata;
}
public void showLink() {
System.out.print(idata + " ");
}
}