一、双向链表的描述

        在单链表中，查找直接后继结点的执行时间为O(1)，而查找直接前驱的执行时间为O(n)。为克服单链表这种单向性的缺点，可以用双向链表。

        在双向链表的结点中有两个指针域，一个指向直接后继，另一个指向直接前驱。

二、双向链表的存储结构

typedef char ElemType[20];  //重定义数据域的数据类型
typedef struct LNode  //定义双向链表存储结构
{
	ElemType data;
	struct LNode *next;
	struct LNode *prev;
}*DoubleLink;

三、双向链表的操作

2.1 双向链表结点创建

DoubleLink Request_space()  //在堆区申请一个结点空间
{
	DoubleLink node=(DoubleLink)malloc(sizeof(struct LNode));
	if(NULL==node)
		return NULL;
	strcpy(node->data,"");
	node->next=node->prev=NULL;
	return node;
}

2.2 双向链表遍历

int Output(DoubleLink L_list)  //实现输出
{
	if(NULL==L_list)
		return -1;
	DoubleLink p=L_list;
	do
	{
		printf("%s ",p->data);
		p=p->next;
	}while(p!=NULL);
	puts("");
	return 0;
}

2.3 双向链表头插

DoubleLink insert_head(DoubleLink L_list,ElemType value)  //实现头插
{
	DoubleLink node=Request_space();
	if(NULL==node)
		return L_list;  
	strcpy(node->data,value);
	if(NULL!=L_list)
	{
		node->next=L_list;
		L_list->prev=node;
	}
	L_list=node;
	return L_list;
}

2.4 双向链表尾插

DoubleLink insert_rear(DoubleLink L_list,ElemType value)  //实现尾插
{
	DoubleLink node=Request_space();
	strcpy(node->data,value);
	if(NULL==L_list)
		L_list=node;
	else
	{
		DoubleLink rear=L_list;
		while(rear->next!=NULL)
			rear=rear->next;
		rear->next=node;
		node->prev=rear;
	}
	return L_list;
}

2.5 双向链表头删

DoubleLink delete_head(DoubleLink L_list)  //实现头删
{
	if(NULL==L_list)
		return NULL;
	if(L_list->next==NULL)
	{
		free(L_list);
		L_list=NULL;
	}
	else
	{
		DoubleLink p=L_list->next;
		strcpy(L_list->data,p->data);
		L_list->next=p->next;
		if(p->next!=NULL)
			p->next->prev=L_list;
		free(p);
		p=NULL;
	}
	return L_list;
}

2.6 双向链表尾删

DoubleLink delete_rear(DoubleLink L_list)  //实现尾删
{
	if(NULL==L_list)
		return NULL;
	if(L_list->next==NULL)
	{
		free(L_list);
		L_list=NULL;
	}
	else
	{
		DoubleLink rear=L_list;
		while(rear->next!=NULL)
			rear=rear->next;
		rear->prev->next=NULL;
		free(rear);
		rear=NULL;
	}
	return L_list;
}

2.7 计算双向链表长度

int len_Llist(DoubleLink L_list)  //计算双向链表长度
{
	int count=0;
	if(NULL==L_list)
		return -1;
	while(L_list!=NULL)
	{
		count++;
		L_list=L_list->next;
	}
	return count;
}

2.8 双向链表逆置

DoubleLink rev_list(DoubleLink L_list)  //双向链表逆置
{
	if(NULL==L_list||L_list->next==NULL)
		return L_list;
	int len=len_Llist(L_list)-1;
	DoubleLink p=L_list->next;
	L_list->next=NULL;
    L_list->prev=L_list->next;
	for(int i=0;i<len;i++)
	{
		DoubleLink q=p;
		p=p->next;
		q->prev=q->next;
		q->next=L_list;
		L_list=q;
	}
	return L_list;
}

四、多文件编辑实现双向链表操作

头文件 head.h

#ifndef __HEAD_H__
#define __HEAD_H__

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

typedef char ElemType[20];  //重定义数据域的数据类型
typedef struct LNode  //定义双向链表存储结构
{
	ElemType data;
	struct LNode *next;
	struct LNode *prev;
}*DoubleLink;

DoubleLink Request_space();  //在堆区申请一个结点空间
int Output(DoubleLink L_list);  //实现输出
DoubleLink insert_head(DoubleLink L_list,ElemType value);  //实现头插
DoubleLink insert_rear(DoubleLink L_list,ElemType value);  //实现尾插
DoubleLink delete_head(DoubleLink L_list);  //实现头删
DoubleLink delete_rear(DoubleLink L_list);  //实现尾删
DoubleLink rev_list(DoubleLink L_list);  //双向链表逆置

#endif

自定义函数 fun.c

#include "head.h"
DoubleLink Request_space()  //在堆区申请一个结点空间
{
	DoubleLink node=(DoubleLink)malloc(sizeof(struct LNode));
	if(NULL==node)
		return NULL;
	strcpy(node->data,"");
	node->next=node->prev=NULL;
	return node;
}
int Output(DoubleLink L_list)  //实现输出
{
	if(NULL==L_list)
		return -1;
	DoubleLink p=L_list;
	do
	{
		printf("%s ",p->data);
		p=p->next;
	}while(p!=NULL);
	puts("");
	return 0;
}

DoubleLink insert_head(DoubleLink L_list,ElemType value)  //实现头插
{
	DoubleLink node=Request_space();
	if(NULL==node)
		return L_list;  
	strcpy(node->data,value);
	if(NULL!=L_list)
	{
		node->next=L_list;
		L_list->prev=node;
	}
	L_list=node;
	return L_list;
}

DoubleLink insert_rear(DoubleLink L_list,ElemType value)  //实现尾插
{
	DoubleLink node=Request_space();
	strcpy(node->data,value);
	if(NULL==L_list)
		L_list=node;
	else
	{
		DoubleLink rear=L_list;
		while(rear->next!=NULL)
			rear=rear->next;
		rear->next=node;
		node->prev=rear;
	}
	return L_list;
}

DoubleLink delete_head(DoubleLink L_list)  //实现头删
{
	if(NULL==L_list)
		return NULL;
	if(L_list->next==NULL)
	{
		free(L_list);
		L_list=NULL;
	}
	else
	{
		DoubleLink p=L_list->next;
		strcpy(L_list->data,p->data);
		L_list->next=p->next;
		if(p->next!=NULL)
			p->next->prev=L_list;
		free(p);
		p=NULL;
	}
	return L_list;
}

DoubleLink delete_rear(DoubleLink L_list)  //实现尾删
{
	if(NULL==L_list)
		return NULL;
	if(L_list->next==NULL)
	{
		free(L_list);
		L_list=NULL;
	}
	else
	{
		DoubleLink rear=L_list;
		while(rear->next!=NULL)
			rear=rear->next;
		rear->prev->next=NULL;
		free(rear);
		rear=NULL;
	}
	return L_list;
}
int len_Llist(DoubleLink L_list)  //计算双向链表长度
{
	int count=0;
	if(NULL==L_list)
		return -1;
	while(L_list!=NULL)
	{
		count++;
		L_list=L_list->next;
	}
	return count;
}
DoubleLink rev_list(DoubleLink L_list)  //双向链表逆置
{
	if(NULL==L_list||L_list->next==NULL)
		return L_list;
	int len=len_Llist(L_list)-1;
	DoubleLink p=L_list->next;
	L_list->next=NULL;
	for(int i=0;i<len;i++)
	{
		DoubleLink q=p;
		p=p->next;
		q->prev=q->next;
		q->next=L_list;
		L_list=q;
	}
	return L_list;
}

主函数 main.c

#include "head.h"
int main(int argc, const char *argv[])
{
	DoubleLink L_list=NULL;  //定义头指针变量,注意定义时一定要指向NULL
	int n;            //定义循环输入次数
	ElemType value;   //定义数据域元素
	int seat;  //定义元素位置

	printf("please enter n:");
	scanf("%d",&n);

 	for(int i=0;i<n;i++)  //头插
	{
		printf("please enter a value:");
		scanf("%s",value);
		L_list=insert_head(L_list,value);
	}
	
	for(int i=0;i<n;i++)  //尾插
	{	
		printf("please enter a value:");
		scanf("%s",value);
		L_list=insert_rear(L_list,value);
	}

	L_list=delete_head(L_list);  //头删
	L_list=delete_rear(L_list);  //尾删
	Output(L_list);


	L_list=rev_list(L_list);  //双向链表逆置
	Output(L_list);

	return 0;
}