数据结构—双向链表

双向链表也叫双链表，是链表的一种，它的每个数据结点中都有两个指针，分别指向直接后继和直接前驱。所以，从双向链表中的任意一个结点开始，都可以很方便地访问它的前驱结点和后继结点，时间复杂度为O(1)。

双链表具有以下优点：

1、删除单链表中的某个结点时，一定要得到待删除结点的前驱，得到该前驱有两种方法，第一种方法是在定位待删除结点的同时一路保存当前结点的前驱。第二种方法是在定位到待删除结点之后，重新从单链表表头开始来定位前驱。尽管通常会采用方法一。但其实这两种方法的效率是一样的，指针的总的移动操作都会有2*i次。而如果用双向链表，则不需要定位前驱结点。因此指针总的移动操作为i次。

2、查找时也一样，我们可以借用二分法的思路，从head（首节点）向后查找操作和last（尾节点）向前查找操作同步进行，这样双链表的效率可以提高一倍。

可是为什么市场上单链表的使用多于双链表呢？

从存储结构来看，每个双链表的节点要比单链表的节点多一个指针，而长度为n就需要 n*length（这个指针的length在32位系统中是4字节，在64位系统中是8个字节） 的空间，这在一些追求时间效率不高应用下并不适应，因为它占用空间大于单链表所占用的空间；这时设计者就会采用以时间换空间的做法，这时一种工程总体上的衡量。

代码实现：

#define _CRT_SECURE_NO_WARNINGS
#include "iostream"

using namespace std;

typedef struct Person  //数据
{
	int number;
	char name[32];
}DATATYPE;

typedef struct Node	//结点
{
	DATATYPE data;
	Node *pri;
	Node *next;
}linkNode;

typedef struct List		//链表
{
	linkNode *head;
	int clen;
}linkLIst;

//链表初始化
linkLIst * initList()
{
	linkLIst *tmp = new linkLIst;
	if (NULL == tmp)
	{
		perror("list_init Linklist");
		exit(1);
	}

	tmp->clen = 0;
	tmp->head = NULL;
	return tmp;
}

//头插
int insertHead(linkLIst *mlist,DATATYPE *data)
{
	linkNode *newnode = new linkNode;	//创建要添加的结点
	newnode->next = NULL;
	newnode->pri  = NULL;

	if (NULL == newnode)
	{
		perror("insertHead newnode");
		return 1;
	}
	
	newnode->data = *data;
	newnode->next = mlist->head;

	mlist->head = newnode;
	mlist->clen++;

	return 0;
}

//尾插
int insertTail(linkLIst *mlist, DATATYPE *data)
{
	linkNode *newnode = new linkNode;		//创建要添加的结点
	if (NULL == newnode)
	{
		perror("insrtTail newnode");
		return 1;
	}

	newnode->data = *data;
	newnode->next = NULL;
	newnode->pri = NULL;

	if (NULL == mlist->head)
	{
		mlist->head = newnode;
	}
	else
	{
		linkNode* tmp = mlist->head;

		while (tmp->next)
		{
			tmp = tmp->next;
		}

		tmp->next = newnode;
		newnode->pri = tmp;
	}	
	
	mlist->clen++;
	return 0;
}

//查找
linkNode* findList(linkLIst *mlist,char *pname)
{
	linkNode *tmp = mlist->head;
	while (tmp)
	{
		if (0 == strcmp(tmp->data.name,pname))
		{
			return tmp;
		}
		tmp = tmp->next;
	}
	return NULL;
}

//删除元素
int delList(linkLIst *mlist, char *pname)
{
	if (NULL == mlist->head)			//判断链表是否为空
	{
		cout << "链表已为空" << endl;
		return 1;
	}

	if (NULL == mlist->head->next)				//判断是否只有表头
	{
		if (0 == strcmp(mlist->head->data.name,pname))
		{
			delete mlist->head;
			mlist->head = NULL;
		}
		else
		{
			cout << "未查到。。。" << endl;
			return 1;
		}
	}
	else
	{
		linkNode *p = mlist->head;
		linkNode *q = mlist->head;

		while (p)
		{
			if (0 == strcmp(p->data.name,pname))		
			{
				if (p == q)					//判断是否为表头
				{
					mlist->head = p->next;
					p->next->pri = NULL;
				}
				else
				{
					q->next = p->next;
					p->next->pri = q;
				}
				delete p;
				p = NULL;
				mlist->clen--;
				return 0;
			}
			else
			{
				q = p;
				p = p->next;			
			}
		}
	}
	cout << "未找到要删除的元素。。。" << endl;
	return 1;
}

//链表逆序
int reverseList(linkLIst *mlist)
{
	linkNode *p = mlist->head->next;
	linkNode *q = mlist->head;

	while (p)
	{
		q->next = p->next;
		if (p->next)
		{
			p->next->pri = q;
		}
		p->next = mlist->head;
		p->pri = NULL;
		mlist->head = p;
		p = q->next;
	}

	return 0;
}

//链表排序
int sortList(linkLIst *mlist)
{
	if (NULL == mlist->head)
	{
		cout << "链表为空" << endl;
		return 1;
	}

	if (NULL == mlist->head->next)
	{
		cout << "链表只有一个表头" << endl;
		return 0;
	}
	else
	{
		linkNode *p, *q, temp;
		q = mlist->head;
		while (q->next != NULL)
		{
			p = q->next;
			while (p != NULL)
			{
				if (p->data.number < q->data.number)
				{
					temp.data = p->data;
					p->data = q->data;
					q->data = temp.data;
				}
				p = p->next;
			}
			q = q->next;
		}	
	}

	cout << "排序完成" << endl;
	return 0;
}

//修改链表元素
int updataList(linkLIst *mlist,char *oldname,char *newname)
{
	linkNode *temp;
	temp = findList(mlist,oldname);
	strcpy(temp->data.name, newname);
	return 0;
}

//销毁链表
int destoryList(linkLIst *mlist)
{
	linkNode *tmp = mlist->head;
	while (mlist->head)			//逐个删除每个结点
	{
		tmp = mlist->head;
		mlist->head = tmp->next;
		delete tmp;
	}

	tmp = NULL;
	delete mlist;
	mlist = NULL;
	cout << "释放完成" << endl;
	return 0;
}

//打印链表
int showList(linkLIst *mlist)
{
	linkNode *tmp = mlist->head;
	while (tmp)
	{
		cout << "ID:" << tmp->data.number << " " << "名字：" << tmp->data.name << endl;
		tmp = tmp->next;
	}

	return 0;
}

int main()
{
	linkLIst *m_list = initList();
	DATATYPE data1 = { 1,"刘德华" };
	DATATYPE data2 = { 0,"周润发" };

	cout << "头插：" << endl;
	insertHead(m_list, &data1);
	insertHead(m_list, &data2);	
	cout << "当前链表长度：" << m_list->clen << endl;
	showList(m_list);

	DATATYPE data3[3] = {
		{2,"张学友"},
		{3,"郭富城"},
		{4,"黎明"}
	};
	DATATYPE data4 = { 1,"周星驰" };
	cout << "尾插：" << endl;

	insertTail(m_list, &data3[0]);
	insertTail(m_list, &data3[1]);
	insertTail(m_list, &data3[2]);
	insertTail(m_list, &data4);

	sortList(m_list);
	cout << "当前链表长度：" << m_list->clen << endl;
	showList(m_list);

	cout << "---------------------------------------------------" << endl;
	cout << "链表逆序：" << endl;
	reverseList(m_list);
	cout << "链表长度：" << m_list->clen << endl;
	showList(m_list);

	cout << "---------------------------------------------------" << endl;
	cout << "查找元素：" << endl;
	linkNode* res = findList(m_list, "刘德华");
	if (NULL != res)
	{
		cout << "ID：" << res->data.number << " " << "姓名：" << res->data.name << endl;
	}
	else
	{
		cout << "未找到。。。" << endl;
	}

	cout << "---------------------------------------------------" << endl;
	cout << "删除元素：" << endl;
	delList(m_list, "刘德华");
	cout << "链表长度：" << m_list->clen << endl;
	showList(m_list);

	cout << "---------------------------------------------------" << endl;
	cout << "修改链表元素：" << endl;
	updataList(m_list, "张学友", "成龙");
	showList(m_list);

	cout << "---------------------------------------------------" << endl;
	cout << "销毁链表：" << endl;
	destoryList(m_list);

	system("pause");	
	return 0;
}