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) {

	}

	@Override
	public Iterator<Integer> iterator() {
		return new Iterator<Integer>() {
			Node p = sentinel.next;

			@Override
			public boolean hasNext() {
				return p != sentinel;
			}

			@Override
			public Integer next() {
				int v = p.value;
				p = p.next;
				return v;
			}
		};
	}

}