struct ListNode {
    int data;
    ListNode* prev;
    ListNode* next;
<pre class='brush:php;toolbar:false;'>// 构造函数
ListNode(int val) : data(val), prev(nullptr), next(nullptr) {}

};

class DoublyLinkedList {
private:
    ListNode* head;
    ListNode* tail;
<p>public:
DoublyLinkedList() : head(nullptr), tail(nullptr) {}</p><pre class='brush:php;toolbar:false;'>~DoublyLinkedList() {
    while (head) {
        ListNode* temp = head;
        head = head->next;
        delete temp;
    }
}

// 在链表末尾插入节点
void append(int val) {
    ListNode* newNode = new ListNode(val);
    if (!head) {
        head = tail = newNode;
        return;
    }
    tail->next = newNode;
    newNode->prev = tail;
    tail = newNode;
}

// 在链表开头插入节点
void prepend(int val) {
    ListNode* newNode = new ListNode(val);
    if (!head) {
        head = tail = newNode;
        return;
    }
    newNode->next = head;
    head->prev = newNode;
    head = newNode;
}

// 删除第一个值为val的节点
bool remove(int val) {
    ListNode* current = head;
    while (current) {
        if (current->data == val) {
            if (current->prev) {
                current->prev->next = current->next;
            } else {
                head = current->next;  // 当前是头节点
            }

            if (current->next) {
                current->next->prev = current->prev;
            } else {
                tail = current->prev;  // 当前是尾节点
            }

            delete current;
            return true;
        }
        current = current->next;
    }
    return false;  // 未找到
}

// 打印链表（正向）
void printForward() const {
    ListNode* current = head;
    while (current) {
        <strong>std::cout << current->data << " <-> "; </strong>
        current = current->next;
    }
    <strong>std::cout << "nullptr" << std::endl;</strong>
}

// 打印链表（反向）
void printBackward() const {
    ListNode* current = tail;
    while (current) {
        <strong>std::cout << current->data << " <-> "; </strong>
        current = current->prev;
    }
    <strong>std::cout << "nullptr" << std::endl;</strong>
}

};

int main() {
    DoublyLinkedList dll;
    dll.append(10);
    dll.append(20);
    dll.prepend(5);
    dll.printForward();   // 输出: 5 <-> 10 <-> 20 <-> nullptr
    dll.printBackward();  // 输出: 20 <-> 10 <-> 5 <-> nullptr
    dll.remove(10);
    dll.printForward();   // 输出: 5 <-> 20 <-> nullptr
    return 0;
}