双向环形链表带哨兵，这时哨兵既作为头，也作为尾。

package com.tfq.arithmetic.linkedlist;import java.util.Iterator;/*** @author: fqtang* @date: 2024/05/22/8:40* @description: 环形链表*/
public class DoublyLinkedListSentinel implements Iterable<Integer> {private static class Node {Node prev;//上一个节点指针int value;//值Node next;//下一个节点指针public Node(Node prev, int value, Node next) {this.prev = prev;this.next = next;this.value = value;}}/*** 哨兵对象*/private Node sentinel = new Node(null, -1, null);public DoublyLinkedListSentinel() {sentinel.prev = sentinel;sentinel.next = sentinel;}/*** 添加到第一个** @param value -待添加值*/public void addFirst(int value) {Node a = sentinel;Node b = sentinel.next;Node added = new Node(a, value, b);a.next = added;b.prev = added;}/*** 向索引位置插值** @param index 索引* @param value 待插入值*/public void insert(int index, int value) {Node findNode = findByIndex(index);Node last = findNode.prev;Node next = findNode;Node added = new Node(last, value, next);last.next = added;next.prev = added;}/*** 添加到最后一个** @param value -待添加值*/public void addLast(int value) {Node a = sentinel.prev;Node b = sentinel;Node added = new Node(a, value, b);a.next = added;b.prev = added;}/*** 删除第一个元素*/public void removeFirst() {Node removed = sentinel.next;if(removed == sentinel) {throw new IllegalArgumentException("不能删除哨兵节点，非法!");}Node last = sentinel;Node next = removed.next;last.next = next;next.prev = last;}/*** 删除最后一个元素*/public void removeLast() {Node removed = sentinel.prev;if(removed == sentinel) {throw new IllegalArgumentException("不能删除哨兵节点，非法!");}Node last = removed.prev;Node next = sentinel;last.next = next;next.prev = last;}/*** 根据索引删除指定元素** @param value 待删除索引*/public void removeByValue(int value) {Node remove = findByValue(value);if(remove != null) {Node last = remove.prev;Node next = remove.next;last.next = next;next.prev = last;}}public Node findByValue(int value) {/* //方法一for(Node p = sentinel.next; p != sentinel; p = p.next) {if(p.value == value) {return p;}}*///方法二Node p = sentinel.next;while(p != sentinel) {if(p.value == value) {return p;}p = p.next;}return null;}public Node findByIndex(int index) {/* //方法一int i=0;for(Node p = sentinel.next; p != sentinel; p = p.next,i++) {if(p.value == value) {return p;}}*///方法二int tIndex=index -1;if(tIndex<-1){throw new IllegalArgumentException(String.format("index [%d] 不合法%n", index));}Node p = sentinel.next;int i = 0;while(p != sentinel) {if(i == tIndex) {return p;}p = p.next;i++;}return null;}/*** 根据索引删除指定元素** @param index 待删除索引*/public void remove(int index) {}@Overridepublic Iterator<Integer> iterator() {return new Iterator<Integer>() {Node p = sentinel.next;@Overridepublic boolean hasNext() {return p != sentinel;}@Overridepublic Integer next() {int v = p.value;p = p.next;return v;}};}}