双向链表C++。在函数后插入不起作用

#include <iostream>

using namespace std;

// 双向链表节点类
class ListNode {
public:
    int val;
    ListNode* prev;
    ListNode* next;

    ListNode(int value) {
        val = value;
        prev = nullptr;
        next = nullptr;
    }
};

// 双向链表类
class DoublyLinkedList {
public:
    ListNode* head;
    ListNode* tail;

    DoublyLinkedList() {
        head = nullptr;
        tail = nullptr;
    }

    // 后插入操作
    void insertAfter(int value, ListNode* prevNode) {
        if (prevNode == nullptr) {
            cout << "Previous node is null." << endl;
            return;
        }

        ListNode* newNode = new ListNode(value);
        newNode->prev = prevNode;
        newNode->next = prevNode->next;
        if (prevNode->next != nullptr) {
            prevNode->next->prev = newNode;
        }
        prevNode->next = newNode;
        if (tail == prevNode) {
            tail = newNode;
        }
    }

    // 打印链表元素
    void printList() {
        ListNode* currentNode = head;
        while (currentNode != nullptr) {
            cout << currentNode->val << " ";
            currentNode = currentNode->next;
        }
        cout << endl;
    }
};

int main() {
    DoublyLinkedList dll;

    // 创建节点
    ListNode* node1 = new ListNode(1);
    ListNode* node2 = new ListNode(2);
    ListNode* node3 = new ListNode(3);

    // 构建链表
    dll.head = node1;
    node1->next = node2;
    node2->prev = node1;
    node2->next = node3;
    node3->prev = node2;
    dll.tail = node3;

    cout << "初始链表：" << endl;
    dll.printList();

    // 后插入节点
    cout << "后插入节点后的链表：" << endl;
    dll.insertAfter(4, node2);
    dll.printList();

    return 0;
}