将项目添加到双向链表的背面时遇到问题

我是双向链表的新手。 我正在处理几段代码：一个函数将项目添加到前面，另一个函数将项目添加到后面，以及分别从前到后和从后到前输出链表的正向和反向显示方法。
我在输出中遇到了一个错误，我试图理解。 似乎我的addFront和显示功能正在工作，但我的addBack可能是我的错误所在。 我在这篇文章的底部发布了我的输出是什么以及它显示的内容。这是我到目前为止的代码。

#include<iostream>
using namespace std;
class doubleLinkedList
{
private:
    class node
    {
    public:
        int data;
        node* next;
        node* prev;
        node(int x)
        {
            data = x;
            next = NULL;
            prev = NULL;
        }
    };
public:
    node* head;
    node* tail;
    int count;

    doubleLinkedList();        //default constructor
    ~doubleLinkedList();      //destructor
    void displayForward();    //display items from front to back
    void displayBackward();   //display items from back to front
    void addFront(int);      //add item to front of linked list
    void addBack(int);       //add item to back of linked list
    int removeFront();       //remove item from front of linked list
    int removeBack();        //remove item from back of linked list
};
//constructor
doubleLinkedList::doubleLinkedList(){
    head = tail = NULL;
    count = 0;
}

//destructor
doubleLinkedList::~doubleLinkedList(){
    node* current = head;
    while(current != NULL)
    {
        node* previous = current;
        current = current->next;
        delete previous;
    }
    head = tail = NULL;
    count = 0;
}

//display items in linked list from front to back
void doubleLinkedList::displayForward(){
    node* pCurrent = head;
    while (pCurrent != NULL)
    {
        cout << pCurrent->data << " ";
        pCurrent = pCurrent->next;
    }
    cout << count;
}

//display items in linked list from back to front
void doubleLinkedList::displayBackward(){
    node* pCurrent = tail;
    while (pCurrent != NULL)
    {
        cout <<pCurrent->data << " ";
        pCurrent = pCurrent->prev;
    }
    cout << count;
}

//add item to front of linked list
void doubleLinkedList::addFront(int x){
    node* n = new node(x);
    n->next = head;
    n->prev = NULL;
    if (head != NULL)
        head->prev = n;
        head = n;
        count++;
    if (tail == NULL)
        tail = n;
}
void doubleLinkedList::addBack(int x){
    node* n = new node(x);
    n->next = NULL;
    n->prev = tail;
    tail = n;
    count++;
}

Code:////////////////////

int main()
{
    doubleLinkedList list;
    list.addBack(40);
    list.addBack(50);
    list.addBack(60);
    list.addFront(30);
    list.addFront(20);
    list.addBack(70);
    list.addBack(80);
    list.addFront(10);
    list.displayForward();  //10 20 30 8   (the 8 value is the count/size i'm keeping track of)
        cout << endl;
    list.displayBackward(); //80 70 60 50 40 8
        cout << endl;
        system("pause");
        return 0;
        }

我的输出应显示 10 20 30 40 50 60 70 80和 80 70 60 50 40 30 20 10
而是我的 displayForward 显示我添加到前面的项目，我的 displayBackward 显示我添加到后面的项目。