环形链表的学习

package DoubleLinkedList;

public class linkedListSentinel {
    private static class Node {
        Node prev;
        int value;
        Node next;

        public Node(Node prev, int value, Node next) {
            this.prev = prev;
            this.value = value;
            this.next = next;
        }
    }

    private Node sentinel = new Node(null, -1, null);

    public linkedListSentinel() {
        sentinel.prev = sentinel;
        sentinel.next = sentinel;
    }

    //添加第一个
    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;
    }

    //添加最后一个
    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
    public void removeFirst() {
        Node removed = sentinel.next;
        if (removed == sentinel) {
            illegalIndex(0);
        }
        Node a = sentinel;
        Node b = removed.next;
        a.next = b;
        b.prev = a;
    }

    //删除最后一个
    public void removeLast() {
        Node removed = sentinel.prev;
        if (removed == sentinel) {
            illegalIndex(0);
        }
        Node a = removed.prev;
        Node b = sentinel;
        a.next = b;
        b.prev = a;
    }

    //遍历
    public void loop() {
        for (Node p = sentinel.next; p != sentinel; p = p.next) {
            System.out.println(p.value);
        }
    }

    //根据值删除
    public void removeByValue(int value) {
        Node removed = findByValue(value);
        if (removed == null) {
            return;
        }
        Node a = removed.prev;
        Node b = removed.next;
        a.next = b;
        b.prev = a;
    }

    private Node findByValue(int value) {
        Node p = sentinel.next;
        while (p != sentinel) {
            if (p.value == value) {
                return p;
            }
            p = p.next;
        }
        return null;
    }

    //抛出异常
    private static void illegalIndex(int index) {
        throw new IllegalArgumentException(String.format("index[%d] 不合法%n", index));
    }
}