前言

在双向链表之前，如果需要查看单链表来复习一下，链接在这里：

http://t.csdnimg.cn/Ib5qS

1.双向链表

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1.1 链表的分类

实际中链表的结构非常多样，以下情况组合起来就有8种链表结构：

1.1.1 单向或者双向

1.1.2 带头或者不带头

1.1.3 循环或者非循环

虽然有这么多的链表的结构，但是我们实际中最常用还是两种结构：

1. 无头单向非循环链表：结构简单，一般不会单独用来存数据。实际中更多是作为其他数据结构的子结构，如哈希桶、图的邻接表等等。另外这种结构在笔试面试中出现很多。

2. 带头双向循环链表：结构最复杂，一般用在单独存储数据。实际中使用的链表数据结构，都是带头双向循环链表。另外这个结构虽然结构复杂，但是使用代码实现以后会发现结构会带来很多优势，实现反而简单了，今天我们就来实现这种代码。

1.2 双向链表的实现

DList.c

#define _CRT_SECURE_NO_WARNINGS 1
#include "DList.h"

LTNode* BuyLTNode(LTDataType x)
{
	LTNode* node = (LTNode*)malloc(sizeof(LTNode));
	if (node == NULL)
	{
		perror("Malloc fail");
		exit(-1);
	}
	node->data = x;
	node->next = NULL;
	return node;
}
LTNode* LTInit()
{
	LTNode* phead  = BuyLTNode(-1);
	phead->next = phead;
	phead->prev = phead;
	return phead;
}

void LTPrint(LTNode * phead)
{
	assert(phead);

	printf("phead<=>");
	LTNode* cur = phead->next;
	while (cur != phead)
	{
		printf("%d<=>", cur->data);
		cur = cur->next;
	}
	printf("\n");
}



void LTPushBack(LTNode* phead, LTDataType x)
{
	assert(phead);

	//LTNode* tail = phead->prev;
	//LTNode* newnode = BuyLTNode(x);

	//newnode->prev = tail;
	//tail->next = newnode;

	//newnode->next = phead;
	//phead->prev = newnode;

	LTInsert(phead->prev, x);

}

void LTPopBack(LTNode* phead)
{
	LTNode* del = NULL;
	//assert(phead);
	//if (phead->prev != phead)//链表指向自己，说明为空
	//{
	//	del = phead->prev;
	//	phead->prev = phead->prev->prev;
	//	phead->prev->next = phead;
	//}
	//else
	//	printf("链表为空，无需尾删");
	//free(del);
	LTErase(phead->prev);
}

void LTPushFront(LTNode* phead, LTDataType x)
{
	assert(phead);


	//newnode->next = phead->next;	//先改变新插入的值，以免链表断开
	//newnode->prev = phead;

	//phead->next->prev = newnode;	//改变原本第二个节点的值
	//phead->next = newnode;			//改变为第一个节点
	//更稳妥的办法：双指针

	//LTNode* newnode = BuyLTNode(x);
	//LTNode* first = phead->next;
	//phead->next = newnode;
	//newnode->prev = phead;
	//newnode->next = first;
	//first->prev = newnode;

	LTInsert(phead->next, x);
}
//头删
void LTPopFront(LTNode* phead)
{
	assert(phead);
	assert(phead->next != phead);

	//LTNode* first = phead->next;
	//LTNode* second = first->next;

	//free(first);

	//phead->next = second;
	//second->prev = phead;

	LTErase(phead->next);
}

int LTSize(LTNode* phead)
{
	assert(phead);

	int size = 0;
	LTNode* cur = phead->next;
	while (cur != NULL)
	{
		size++;
		cur = cur->next;
	}
	return size;
}

void LTInsert(LTNode* pos, LTDataType x)
{
	assert(pos);

	LTNode* posPrev = pos->prev;
	LTNode* newnode = BuyLTNode(x);

	posPrev->next = newnode;
	newnode->prev = posPrev;
	newnode->next = pos;
	pos->prev = newnode;
	  
}
//删除pos位置
void LTErase(LTNode* pos)
{
	assert(pos);
	LTNode* posPrev = pos->prev;
	LTNode* posNext = pos->next;

	free(pos);

	posPrev->next = posNext;
	posNext->prev = posPrev;
}

//寻找值
LTNode* LTFind(LTNode* phead, LTDataType x)
{
	assert(phead);
	assert(phead->next);
	LTNode* pos = phead->next;
	while (pos)
	{
		if (pos->data == x)
		{
			return pos;
		}
		pos = pos->next;
	}
}

//删除表
void LTDestroy(LTNode* phead)
{
	assert(phead);
	LTNode* cur = phead->next;
	while (cur != phead)
	{
		LTNode* next = cur->next;
		free(cur);

		cur = next;
	}
}

DList.h

#pragma once
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>

typedef int LTDataType;
typedef struct ListNode
{
	struct ListNode* next;
	struct ListNode* prev;
	LTDataType data;
}LTNode;
//申请空间
LTNode* BuyLTNode(LTDataType x);
//初始化指针
LTNode* LTInit();
//打印
void LTPrint(LTNode* phead);
//尾插
void LTPushBack(LTNode* phead, LTDataType x);
//尾删
void LTPopBack(LTNode* phead);
//头插
void LTPushFront(LTNode* phead, LTDataType x);
//头删
void LTPopFront(LTNode* phead);
//记录个数
int LTSize(LTNode* phead);
//pos之前插入
void LTInsert(LTNode* pos, LTDataType x);
//删除pos位置
void LTErase(LTNode* pos);
//寻找值
LTNode* LTFind(LTNode* phead, LTDataType x);
//删除表
void LTDestroy(LTNode* phead);

test.c

#define _CRT_SECURE_NO_WARNINGS 1
#include "DList.h"

void TestList1()
{
	LTNode* plist = NULL;
	plist = LTInit();
	LTPushBack(plist, 1);
	LTPushBack(plist, 2);
	LTPushBack(plist, 3);
	LTPushBack(plist, 4);
	LTPushBack(plist, 5);
	LTPrint(plist);

	LTPopBack(plist);
	LTPrint(plist);

	LTPushFront(plist, 99);
	LTPrint(plist);

	LTPopFront(plist);
	LTPrint(plist);

	LTNode* testlist = LTFind(plist, 5);
	LTPrint(testlist);

}
int main()
{
	TestList1();

}