数据结构<二>双向链表

#include<iostream>
using namespace std;
struct Node{
  int data;
  Node* prev;
  Node* next;
};
class List{
  private:
    Node* head;
  public:
    List();
    ~List();
    int Length();
    bool Empty();
    void clear();
    void push_front(int x);
    void push_back(int x);
    void pop_front();
    void pop_back();
    void print();     
};
List::List(){
  head = new Node;
  head->prev = head->next = head;
}
List::~List(){
  clear();
  delete head;
}
int List::Length(){
  Node *q = head->next;
  int k = 0;
  while (q != head) {
    k++;
    q = q->next;
  }
  return k;
}
bool List::Empty(){
  return head->next == head;
}
void List::push_front(int x){
  Node* q = new Node;
  q->data = x;
  q->prev = head;
  q->next = head->next;
  head->next->prev = q;
  head->next = q;
}
void List::push_back(int x){
  Node* q = new Node;
  q->data = x;
  q->prev = head->prev;
  q->next = head;
  head->prev->next = q;
  head->prev = q;
}
void List::pop_front(){
  if (Empty()) return ;
  Node *q = head->next;
  head->next = q->next;
  q->next->prev = head;
  delete q;
}
void List::pop_back(){
  if (Empty()) return ;
  Node *q = head->prev;
  head->prev = q->prev;
  q->prev->next = head;
  delete q;
}
void List::clear() {
  Node *p = head->next; 
  while(p != head) {
    Node *q = p; 
    p = p->next; 
    delete q; 
  } 
  head->next = head;
  head->prev = head;
}
void List::print() {
  Node *q = head->next;
  while (q != head) {
    cout<<q->data<<" ";
    q = q->next; 
  }
  cout<<endl;
}
int main() {
  List l;
  l.push_back(2);  
  l.push_front(3);  
  l.push_back(4);
  l.push_front(5);
  l.push_back(6);
  l.pop_front();
  l.print();
  cout<<l.Length()<<endl;
  return 0;
}