文章目录

• 带头结点的双向循环链表和单链表的区别
• 面试题：写一个双向链表
• 代码实现

带头结点的双向循环链表和单链表的区别

  相比我们讲的无头单向非循环的单链表结构，他的插入和删除都是O(1)，也不需要增容，浪费空间换取时间，但是需要申请和释放空间。

面试题：写一个双向链表

思路
  主要实现函数ListInsert，ListFind，ListErase,其他的函数如头插，尾插，头删，尾删等函数可以用以上几个关键的函数代替实现。

代码实现

void ListPushFront(ListNode* phead, LTDataType x)
{
	assert(phead);
	ListNode* node = (ListNode*)malloc(sizeof(ListNode*));
	node->val = x;

	ListNode* next = head->next;
	node->next = next;
	head->next = node;
	next->prev = node;
	node->prev = head;
}

void ListPopBack(ListNode* phead)
{
	assert(phead);
	assert(phead->next != head);

	ListNode* tail = phead->prev;
	ListNode* tailPrev = tail->prev;

	free(tail);
	tailPrev->next = phead;
	phead->prev = tailPrev;
}

void ListPopFront(ListNode* phead)
{
	assert(phead);
	assert(phead->next != head);

	ListNode* node = phead->next;
	ListNode* next = node->next;
	head->next = next;
	next->prev = head;
	free(node);
}

void ListInsert(ListNode* phead, LTDataType x)
{
	assert(pos);
	ListNode* newNode = BuyListNode(x);
	ListNode posPrev = pos->prev;

	posPrev->next = newNode;
	newNode->next = pos;
	pos->prev = newNode;
	newNode->prev = posPrev;
}

void ListFind(ListNode* phead, LTDataType x)
{
	assert(phead);

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

void ListErase(ListNode* pos)
{
	assert(pos);
	//pos不能是phead，否则头结点没了
	//assert(pos != phead);

	ListNode* posPrev = pos->prev;
	ListNode* posNext = pos->next;

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

void ListDestory(ListNode* phead)
{
	ListNode* cur = phead->next;
	while (cur != phead)
	{
		ListNode* next = cur->next;
		ListErase(cur);
		cur = next;
	}
	free(phead);0
}