一、双向链表是什么？
双向链表也叫双链表，是链表的一种，它的每个数据结点中都有两个指针，分别指向直接后继和直接前驱。所以，从双向链表中的任意一个结点开始，都可以很方便地访问它的前驱结点和后继结点。一般我们都构造双向循环链表。
LinkedList底层就是一个双向链表，我们来实现一个双向链表。
这里多一个尾指针，方便我们对尾插操作从O(n)降到O(1).每个结点多了前驱结点，方便我们对链表进行操作。
二、具体方法实现
定义结点
class ListNode {
int value;
ListNode next;
ListNode prev;
public ListNode(int value) {
this.value = value;
}
}下标访问异常
public class IndexWrongException extends RuntimeException{
public IndexWrongException() {
}
public IndexWrongException(String message) {
super(message);
}
}获取链表长度
//得到单链表的长度
public int size(){
ListNode cur = head;
int count = 0;
while(cur != null) {
cur = cur.next;
count++;
}
return count;
}打印链表
public void display(){
ListNode cur = head;
while (cur != null) {
System.out.print(cur.value+" ");
cur = cur.next;
}
System.out.println();
}清空链表
public void clear(){
if(this.head == null) {
return;
}
ListNode cur = this.head;
while(cur != null) {
ListNode curNext = cur.next;
cur.prev = null;
cur.next = null;
cur = curNext;
}
head = null;
tail = null;
}头插法
public void addFirst(int data){
ListNode node = new ListNode(data);
if(head == null) {
this.head = node;
this.tail = node;
return;
}
node.next = this.head;
this.head.prev = node;
this.head = node;
}尾插法
public void addLast(int data){
ListNode node = new ListNode(data);
if(head == null) {
this.head = node;
this.tail = node;
return;
}
tail.next = node;
node.prev = tail;
tail = node;
}指定位置插入
public void addIndex(int index,int data) throws IndexWrongException{
if(index < 0 || index > size()) {
throw new IndexWrongException("输入下标不合法");
}
ListNode node = new ListNode(data);
if(index == 0) {
addFirst(data);
return;
}
if(index == size()) {
addLast(data);
return;
}
ListNode cur = this.head;
while(index != 0) {
cur = cur.next;
index--;
}
node.next = cur;
cur.prev.next = node;
node.prev = cur.prev;
cur.prev = node;
}查找元素
public boolean contains(int key){
if(head == null) {
return false;
}
ListNode cur = this.head;
while(cur != null) {
if(cur.value == key) {
return true;
}
cur = cur.next;
}
return false;
}删除第一次出现的关键字
public void remove(int key){
ListNode cur = head;
while(cur != head) {
if(cur.value == key) {
if(cur == head) {
head = head.next;
if(head != null) {
head.prev = null;
}else {
tail = null;
}
}else {
cur.prev.next = cur.next;
if(cur.next != null) {
cur.next.prev = cur.prev;
}else {
tail = cur.prev;
tail.next = null;
}
}
return;
}
cur = cur.next;
}
}删除所有值为key的节点
public void removeAllKey(int key){
ListNode cur = head;
while(cur != head) {
if(cur.value == key) {
if(cur == head) {
head = head.next;
if(head != null) {
head.prev = null;
}else {
tail = null;
}
}else {
cur.prev.next = cur.next;
if(cur.next != null) {
cur.next.prev = cur.prev;
}else {
tail = cur.prev;
tail.next = null;
}
}
}
cur = cur.next;
}
}三、完整代码
public class LinkedList {
static class ListNode {
int value;
ListNode next;
ListNode prev;
public ListNode(int value) {
this.value = value;
}
}
ListNode head;
ListNode tail;
//头插法
public void addFirst(int data){
ListNode node = new ListNode(data);
if(head == null) {
this.head = node;
this.tail = node;
return;
}
node.next = this.head;
this.head.prev = node;
this.head = node;
}
//尾插法
public void addLast(int data){
ListNode node = new ListNode(data);
if(head == null) {
this.head = node;
this.tail = node;
return;
}
tail.next = node;
node.prev = tail;
tail = node;
}
//任意位置插入,第一个数据节点为0号下标
public void addIndex(int index,int data) throws IndexWrongException{
if(index < 0 || index > size()) {
throw new IndexWrongException("输入下标不合法");
}
ListNode node = new ListNode(data);
if(index == 0) {
addFirst(data);
return;
}
if(index == size()) {
addLast(data);
return;
}
ListNode cur = this.head;
while(index != 0) {
cur = cur.next;
index--;
}
node.next = cur;
cur.prev.next = node;
node.prev = cur.prev;
cur.prev = node;
}
//查找是否包含关键字key是否在单链表当中
public boolean contains(int key){
if(head == null) {
return false;
}
ListNode cur = this.head;
while(cur != null) {
if(cur.value == key) {
return true;
}
cur = cur.next;
}
return false;
}
//删除第一次出现关键字为key的节点
public void remove(int key){
ListNode cur = head;
while(cur != head) {
if(cur.value == key) {
if(cur == head) {
head = head.next;
if(head != null) {
head.prev = null;
}else {
tail = null;
}
}else {
cur.prev.next = cur.next;
if(cur.next != null) {
cur.next.prev = cur.prev;
}else {
tail = cur.prev;
tail.next = null;
}
}
return;
}
cur = cur.next;
}
}
//删除所有值为key的节点
public void removeAllKey(int key){
ListNode cur = head;
while(cur != head) {
if(cur.value == key) {
if(cur == head) {
head = head.next;
if(head != null) {
head.prev = null;
}else {
tail = null;
}
}else {
cur.prev.next = cur.next;
if(cur.next != null) {
cur.next.prev = cur.prev;
}else {
tail = cur.prev;
tail.next = null;
}
}
}
cur = cur.next;
}
}
//得到单链表的长度
public int size(){
ListNode cur = head;
int count = 0;
while(cur != null) {
cur = cur.next;
count++;
}
return count;
}
public void display(){
ListNode cur = head;
while (cur != null) {
System.out.print(cur.value+" ");
cur = cur.next;
}
System.out.println();
}
public void clear(){
if(this.head == null) {
return;
}
ListNode cur = this.head;
while(cur != null) {
ListNode curNext = cur.next;
cur.prev = null;
cur.next = null;
cur = curNext;
}
head = null;
tail = null;
}
}
