NullPointerException在双向链表实现中的应用

class Node {
    int data;
    Node prev;
    Node next;

    Node(int data) {
        this.data = data;
        this.prev = null;
        this.next = null;
    }
}

class DoublyLinkedList {
    Node head;

    // 在链表尾部添加节点
    void append(int data) {
        Node newNode = new Node(data);
        if (head == null) {
            head = newNode; // 如果链表为空，新节点成为头节点
        } else {
            Node current = head;
            while (current.next != null) {
                current = current.next; // 移动到链表尾部
            }
            current.next = newNode; // 添加新节点
            newNode.prev = current; // 设置新节点的前驱指针
        }
    }

    // 在指定节点后插入新节点
    void insertAfter(Node prevNode, int data) {
        if (prevNode == null) {
            throw new IllegalArgumentException("Previous node cannot be null");
        }
        Node newNode = new Node(data);
        newNode.next = prevNode.next;
        newNode.prev = prevNode;
        prevNode.next = newNode;
        if (newNode.next != null) {
            newNode.next.prev = newNode; // 更新后继节点的前驱指针
        }
    }
}