双向链表

在单链表中，我们有了next指针，这就使得我们要查询的下一个节点的时间复杂度为O（1），可是我们要找的是上一个节点的话，那么最坏的情况就是O（n）了，因为我们每次都要从头开始查找。为了改善查找的时间复杂度，就有了现在的双向链表。

双向链表：就是在单链表的每个节点中加入一个指向其前驱的指针域。所以在双向链表的每个节点中存在两个指针域，一个指向前驱，一个指向后继。

双向链表的实现基本上与单链表的实现一致，就是多了一个指向其前驱的指针，在写的时候要注意其前驱指针。

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

typedef struct DNode
{
	int data;
	struct DNode *pre;
	struct DNode *next;
}DNode,*Dlist;

//初始化
void initList(Dlist list)
{
	assert(list != nullptr);
	list->pre = nullptr;
	list->next = nullptr;
}

//创建一个节点
DNode* create(int val)
{
	DNode * node = (DNode *)malloc(sizeof(DNode));
	assert(node != nullptr);
	node->data = val;
	node->pre = nullptr;
	node->next = nullptr;
	return node;
}

//头插
bool headInsert(Dlist list, int val)
{
	assert(list != nullptr);
	DNode *Get = create(val);
	Get->pre = list;
	Get->next = list->next;
	list->next = Get;
	return true;
}

//尾插
bool tailInsert(Dlist list, int val)
{
	assert(list != nullptr);
	DNode *cur = list;
	while (cur->next != nullptr)		//err
	{
		cur = cur->next;
	}

	DNode *Get = create(val);
	//Get->next = nullptr;
	cur->next = Get;
	Get->pre = cur;
	return true;
}

//长度
int Length(Dlist list)
{
	assert(list != nullptr);
	DNode *cur = list;
	int count = 0;
	while (cur->next != nullptr)
	{
		count++;
		cur = cur->next;
	}
	return count;
}

bool isEmpty(Dlist list)
{
	assert(list != nullptr);
	if (list->next == nullptr)
	{
		return true;
	}
	return false;
}

//pos位置插入
bool insert(Dlist list, int pos, int val)
{
	assert(list != nullptr);
	if (pos > Length(list) && pos < 0)
	{
		return false;
	}

	DNode *cur = list;
	for (int i = 1; i < pos; ++i)
	{
		cur = cur->next;
	}
	DNode *get = create(val);
	get->next = cur->next;
	cur->next = get;
	get->pre = cur;
	return true;
}

//show
void Show(Dlist list)
{
	assert(list != nullptr);
	DNode *cur = list->next;
	while (cur != nullptr)
	{
		printf("%d  ", cur->data);
		cur = cur->next;
	}
	printf("\n");
}

//查询是否存在
DNode* Find(Dlist list, int val)
{
	assert(list != nullptr);
	DNode *cur = list;
	while (cur->next != nullptr)
	{
		if (cur->next->data == val)
		{
			//printf("exist\n");
			return cur;
		}
		cur = cur->next;
	}
	//printf("no\n");
	return nullptr;
}

//值删除
bool Delete(Dlist list, int val)
{
	assert(list != nullptr);
	DNode *p = Find(list, val);
	if (p == nullptr)
	{
		printf("没有该值\n");
		return false;
	}

	DNode *cur = p->next;
	p->next = cur->next;
	if (p->next != nullptr)
	{
		cur->next->pre = p;
		free(cur);
	}
	else 
	{
		free(cur);
	}
	cur = nullptr;
	return true;
}

//pos位置删除
bool posDelete(Dlist list, int pos)
{
	assert(list != nullptr);
	if (pos<0 && pos>Length(list))
	{
		return false;
	}

	DNode *cur = list;
	for (int i = 0; i < pos; ++i)
	{
		cur = cur->next;
	}

	DNode *p = cur->pre;
	p->next = cur->next;
	if (p->next != nullptr)
	{
		cur->next->pre = p;
		free(cur);
	}
	else
	{
		free(cur);
	}
	cur = nullptr;
	return true;
}

//释放
void Destory(Dlist list)
{
	DNode *del = nullptr;
	while (list->next != nullptr)
	{
		del = list->next;
		list->next = del->next;
		free(del);
	}
	del = nullptr;
}

test

int main()
{
	DNode head;
	initList(&head);
	for (int i = 1; i < 10; ++i)
	{
		tailInsert(&head, i);
	}
	Show(&head);
	insert(&head, 3, 99);
	Show(&head);
	posDelete(&head, 10);
	Show(&head);
	int len = Length(&head);
	printf("%d\n", len);
	Destory(&head);
	return 0;
}