// 节点结构体
struct Node {
    int data;
    Node* prev;
    Node* next;
};
// 双向链表结构体
struct DoubleLink {
    Node* head;
    Node* tail;
    int size;
    DoubleLink() {
        head = nullptr;
        tail = nullptr;
        size = 0;
    }
    ~DoubleLink() {
        Node* curr = head;
        while (curr) {
            Node* next = curr->next;
            delete curr;
            curr = next;
        }
    }
    // 判断链表是否为空
    bool is_empty() const {
        ________
    }
};