双向链表的创建（带头节点&不带头节点）

#include <iostream>
using namespace std;
//创建链表类节点
class ListNode
{
public:
	//无参构造函数
	ListNode() 
	{
		pre = nullptr;
		next = nullptr; 
	}
	//有参构造函数
	ListNode(int x)
	{
		val = x;
		pre = nullptr;
		next = nullptr;
	}

	int val;
	ListNode* pre;
	ListNode* next;
};
//不带头节点的双向链表链表创建
ListNode * NoHead()
{
	ListNode * head = nullptr;
	ListNode * p = nullptr;
	for (int i = 0; i < 10; i++)
	{
		ListNode * temp = new ListNode(i);
		if (!head)//若第一次赋值，head为空
		{
			head = temp;
			p = temp;
		}
		else
		{
			temp->pre = p;
			p->next = temp;
			p = p->next;
		}
	}
	return head;
}
//带头节点的双向链表链表创建
ListNode * WithHead()
{
	ListNode * head = new ListNode;
	ListNode * p = head;
	for (int i = 10; i < 20; i++)
	{
		ListNode * temp = new ListNode(i);
		p->next = temp;
		temp->pre = p;
		p = p->next;
	}
	return head;
}
//访问链表
void Visit(ListNode * head)
{
	ListNode * p = head;
	while (p->next)
	{
		cout << p->val << " ";
		p = p->next;
	}
	if (p == head || !p->next)//访问头部第一个节点，或者，尾部最后一个节点
		cout << p->val << endl;
	//带头节点的从后往前循环的时候会访问头节点
	while (p!=head)//保证最后一个输出的是带值的节点
	{
		cout << p->val << " ";
		p = p->pre;
	}
	if (p == head || !p->next)
		cout << p->val << endl;
}

int main()
{
	ListNode * head = NoHead();
	cout << "NoHead" << endl;
	Visit(head);
	head = WithHead();
	cout << "WithHead" << endl;
	Visit(head->next);
	system("pause");
	return 0;
}