初阶数据结构——带头双向循环链表

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，后面我们代码实现了就知道了。

双向链表

List.h

#include
#include
#include
#include

typedef int LTDataType;
typedef struct ListNode
{
	struct ListNode* prev;
	struct ListNode* next;
	LTDataType data;
}LTNode;

//开哨兵位的头节点
LTNode*LTInit();
//头插
void LTPushFront(LTNode* phead, LTDataType x);
//头删
void LTPopFront(LTNode* phead);
//尾插
void LTPushBack(LTNode* phead, LTDataType x);
//尾删
void LTPopBack(LTNode* phead);
//pos之前位置插入
void LTNodeInsert(LTNode* pos, LTDataType x);
//删除pos位置
void LTNodeErase(LTNode* pos);
//打印
void LTPrint(LTNode* phead);
//销毁
void LTDestory(LTNode* phead);
//查找
LTNode* LTFind(LTNode* phead, LTDataType x);

List.c

想要快速实现一个链表，那么复用pos位置的插入删除即可

LTNode* BuyLTNode(LTDataType x)
{
	LTNode* newnode = (LTNode*)malloc(sizeof(LTNode));
	if (newnode == NULL)
	{
		perror("malloc fail");
		return;
	}

	newnode->data = x;
	newnode->next = NULL;
	newnode->prev = NULL;

	return newnode;
}

bool LTEmpty(LTNode* phead)
{
	assert(phead);

	return phead->next == phead;
}

LTNode* LTInit()
{
	LTNode* phead = BuyLTNode(-1);
	phead->prev = phead;
	phead->next = phead;

	return phead;
}

//pos之前位置插入
void LTNodeInsert(LTNode* pos, LTDataType x)
{
	assert(pos);

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

	newnode->prev = prev;
	prev->next = newnode;
	newnode->next = pos;
	pos->prev = newnode;
}

//删除pos位置
void LTNodeErase(LTNode* pos)
{
	assert(pos);
	assert(!LTEmpty(pos));

	LTNode* prev = pos->prev;
	LTNode* next = pos->next;

	prev->next = next;
	next->prev = prev;
	free(pos);
}

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

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

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

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

	//如果复用也可以
	//LTNodeInsert(phead->next, x);
}
//头删
void LTPopFront(LTNode* phead)
{
	assert(phead);
	assert(!LTEmpty(phead));

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

	phead->next = second;
	second->prev = phead;
	free(first);

	//如果复用
	//LTNodeErase(phead->next);
}
//尾插
void LTPushBack(LTNode* phead, LTDataType x)
{
	assert(phead);

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

	//如果复用
	//LTNodeInsert(phead,x);
}
//尾删
void LTPopBack(LTNode* phead)
{
	assert(phead);
	assert(!LTEmpty(phead));

	LTNode* tail = phead->prev;
	LTNode* prevtail = tail->prev;

	prevtail->next = phead;
	phead->prev = prevtail;
	free(tail);

	//如果复用
	//LTNodeErase(phead->prev);
}

LTNode* LTFind(LTNode* phead, LTDataType x)
{
	assert(phead);

	LTNode* cur = phead->next;
	while (cur != phead)
	{
		if (cur->data == x)
		{
			return cur;
		}
		else
		{
			cur = cur->next;
		}
	}
	return NULL;
}

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

	LTNode* cur = phead->next;
	while (cur != phead)
	{
		LTNode* next = cur->next;
		free(cur);
		cur = next;
	}
	free(cur);
	//使用者自己将phead置空
}

Test.c

void Test2()
{
	LTNode* phead = LTInit();
	LTPushFront(phead, 1);
	LTPushFront(phead, 2);
	LTPushBack(phead, 3);
	LTPushBack(phead, 4);
	LTPrint(phead);

	LTPopBack(phead);
	LTPrint(phead);
	LTPopFront(phead);
	LTPopFront(phead);
	LTPrint(phead);

	LTPopBack(phead);
	LTPrint(phead);

	LTDestory(phead);
	phead = NULL;
}

int main()
{
	Test2();
	return 0;
}