这是一个题目，但是后期出题者又说一个tail有问题
由于实在太菜了所以不会写……
请问可以帮忙补全代码片段吗？大概的意思已经注释了
使得程序运行后有如下输出：
I love c++ very much!
I love c++
I
love
c++

代码现在是这样
#include
#include

using namespace std;

//MyList
template <class T> struct ListNode {
ListNode(const T& _data = T()): pre(0), next(0), data(_data){}

ListNode<T>* pre;
ListNode<T>* next;
T data;
};

template <typename T> class MyList {
private:
typedef ListNode<T> Node;
ListNode<T>* head;
ListNode<T>* tail;
class list_iterator{
private:
    Node* ptr;//指向mylist容器中的某个元素的指针
public:
    list_iterator(Node* p=nullptr):ptr(p){}
    //重载*, ++, --, ->等基本操作
    T &operator*() const{
        //返回引用 方便通过*it来修改对象
        return ptr->data;
    }
    Node *operator->() const{
        //重载->运算符
        return ptr;
    }
    list_iterator &operator++(){
        //TODO:迭代器++，使迭代器内置指针向后移动一位

    }
    list_iterator operator++(int){
        //TODO:后置++

    }
    bool operator==(const list_iterator &t) const{
        // TODO:重载==

    }
    bool operator!=(const list_iterator &t) const{
        // TODO:重载!=

    }
};
public:
typedef list_iterator iterator;
MyList():head(new Node) {
    head->next = head;
    head->pre = head;
};
~MyList(){
    //Clear();
    delete head;
    head = NULL;
};

bool empty() { //判断list是否为空
    return head->next == head;
}
void push_back(const T& data) { //尾部插入数据
    Node* newnode = new Node(data);
    if (empty())
    {
        head->next = newnode;
        head->pre = newnode;
        newnode->next = head;
        newnode->pre = head;
    }
    else
    {
        Node* tail = head-> pre;
        tail->next = newnode;
        newnode->next = head;
        newnode->pre = tail;
        head->pre = newnode;
    }
}
void pop_back() { //删除尾部数据
    if (!empty()) {
        Node* del = head->pre;
        Node* tail = del->pre;
        delete del;
        tail->next = head;
        head->pre = tail;
    }
}
void print() { //打印list
    if (!empty()) {
        Node* cur = head->next;
        while (cur!=head) {
            cout << cur->data << ' ';
            cur = cur->next;
        }
        cout << endl;
    }
}
int size() { //返回list大小
    Node* cur = head->next;
    int count = 0;
    while (cur!=head) {
        ++count;
        cur = cur->next;
    }
    return count;
}
//迭代器操作方法
iterator begin() const{
    //TODO：返回链表头部指针
}
iterator end() const{
    //TODO：返回链表尾部指针
}
//其他成员函数 可以自己尝试实现insert/erase
};

int main() {
MyList<string> mylist_str;
mylist_str.push_back("I");
mylist_str.push_back("love");
mylist_str.push_back("c++");
mylist_str.push_back("very");
mylist_str.push_back("much!");
mylist_str.print();
mylist_str.pop_back();
mylist_str.pop_back();
mylist_str.print();
for (MyList<string>::iterator it = mylist_str.begin(); it != mylist_str.end(); it++) {
    cout << *it << endl;
}

return 0;
}
十分感谢！