如何用C++实现双向循环链表

双向循环链表，即每个节点都拥有一前一后两个指针且头尾互链的链表。各种链表的简单区别如下：
单向链表：基本链表；
单向循环链表：不同于单向链表以 NULL 判断链表的尾部，单向循环链表的尾部链接到表头，因此当迭代操作到表头前即是尾部；
双向链表：比单向链表多出指向前一个节点的指针，但实际上使用双向链表时很少使用不循环的；
双向循环链表：相对于单向循环链表，双向循环链表可从头部反向迭代，这在链表长度很大且需要获取、插入或删除靠近链表尾部元素的时候十分高效。单向循环列表只能从表头正向迭代，执行的时间大于从反向迭代。
node.h
复制代码 代码如下:

class Node {
public:
    int element;
    Node *next;
    Node *previous;
    Node(int element, Node *next, Node *previous) {
        this->element = element;
        this->next = next;
        this->previous = previous;
    }
};
linkedlist.h:
#include "node.h“
struct linkedList {
    linkedList();
    void addFirst(int);
    void addLast(int);
    void add(int index, int element);
    int getFirst();
    int getLast();
    int get(int);
    int removeFirst();
    int removeLast();
    int remove(int);
    void iterate();
private:
    Node *header;
    int size;
};
linkedlist.cpp:
#include "linkedlist.h"
#include
using std::cout;

linkedList::linkedList() {
    header = new Node(NULL, NULL, NULL);
    header->next = header;
    header->previous = header;
    size = 0;
}

void linkedList::addFirst(int i) {
    header->next = new Node(i, header->next, header);
    header->next->next->previous = header->next;
    ++size;
}

void linkedList::addLast(int i) {
    header->previous = new Node(i, header, header->previous);
    header->previous->previous->next = header->previous;
    ++size;
}

void linkedList::add(int index, int i) {
    if(index > size || index < 0) {
        cout << "Exception in add(): Index out of bound." << 'n';
    return;
    }
    Node *entry;
    if(index < size / 2) {
        entry = header->next;
        for(int i = 0; i < index; ++i)
            entry = entry->next;
    }
    else {
        entry = header;
        for(int i = size; i > index; --i)
            entry = entry->previous;
    }
    entry->previous->next = new Node(i, entry, entry->previous);
    entry->previous = entry->previous->next;
    ++size;
}

int linkedList::getFirst() {
    if(!size)
        cout << "Exception in getFirst(): List is empty." << 'n';
    return header->next->element;
}

int linkedList::getLast() {
    if(!size)
        cout << "Exception in getLast(): List is empty." << 'n';
    return header->previous->element;
}

int linkedList::removeFirst() {
    int remove = header->next->element;
    header->next->next->previous = header;
    header->next = header->next->next;
    --size;
    return remove;
}

int linkedList::removeLast() {
    int remove = header->previous->element;
    header->previous->previous->next = header;
    header->previous = header->previous->previous;
    --size;
    return remove;
}

void linkedList::iterate() {
    if(!size) {
        cout << "Exception in iterate(): List is empty." << 'n';
        return;
    }
    for(Node *entry = header->next; entry != header; entry = entry->next)
        cout << entry->element << " ";
    cout << 'n';
}