双向链表的结点包含前驱指针和后继指针，队列入队操作是在双向链表尾部添加元素，队列的出队操作是把双向链表的头结点删除，判断队列是否为空只要判断双向链表的头指针是否指向了NULL即可。

# include <iostream>
# include <cstdlib>
using namespace std;

struct queueNode
{
	int val;
	queueNode *pre;
	queueNode *next;
	queueNode(int x):val(x),pre(NULL),next(NULL){}
};

queueNode *head=NULL;
queueNode *tail=NULL;
queueNode *cur=head;
void push(int x)   //入队
{
if(head==NULL)
{
head=new queueNode(x);
cur=head;
tail=head;
}
else 
{
queueNode *tmp=new queueNode(x);
cur->next=tmp;
tmp->pre=cur;
cur=tmp;
tail=cur;
}
}

bool isempty(queueNode *head)  //判断队列是否为空
{
return head==NULL;
}
void pop()    //出队
{
	if(tail==NULL)
		return ;
	queueNode *tmp=head;
	head=head->next;
	if(head)
	head->pre=NULL;
	delete tmp;
}

int front(queueNode *head)  //返回队头元素
{
if(isempty(head))
	return -1;
return head->val;
}

int main()
{
	push(1);
	push(2);
	push(3);
	cout<<front(head)<<endl;
	pop();
	cout<<isempty(head)<<endl;
system("pause");
return 0;
}