剑指offer十五：在O(1)时间删除链表结点

结点定义如下：

typedef struct linklist
{
	int data;
	linklist* next;
}node, *pnode;

 在头部添加结点，生成一个一定长度的链表：

pnode addNodeToHead(pnode head, pnode newnode)
{
	if (head == NULL)
	{
		newnode->next = NULL;
		head = newnode;
		return head;
	}
	newnode->next = head;
	head = newnode;
	return head;
}

通过循环遍历找到待删除结点的方法,复杂度为O(n)：

pnode deleteNode(pnode head, pnode beDeleted)
{
	if (head == NULL || beDeleted == NULL)
		return NULL;

	pnode temp = head;
//被删除的结点为头结点
	if (temp->data == beDeleted->data)
	{
		temp = temp->next;
		return head;
	}

	while (temp->next->data != beDeleted->data)
	{
		temp = temp->next;
		if (temp->next == NULL)//被删除的结点不在该链表中
			return NULL;
	}
	pnode a = temp->next;
	temp->next = a->next;
	free(a);
	return head;
}

通过赋值覆盖间接删除该结点，复杂度为O(1)：

pnode DeleteNodeEffect(pnode head, pnode beDelected)
{
	pnode temp = head;
	if (temp == NULL || beDelected == NULL)
		return NULL;
	//链表中只有一个结点
	if (temp->next == NULL)
	{
		delete temp;
		temp = NULL;
	}
	//被删除结点不在尾部
	if (beDelected->next != NULL)
	{
		pnode Dnext = beDelected->next;
		beDelected->data = Dnext->data;
		beDelected->next = Dnext->next;
		delete Dnext;
	}
	//被删除结点在尾部，就需要顺序遍历
	if (beDelected->next == NULL)
	{
		pnode temp = head;
		while (temp->next != beDelected)
		{
			temp = temp->next;
		}
		temp->next = NULL;
		delete beDelected;
	}
	return head;
}

完整代码：

#include<iostream>
using namespace std;

//定义链表结点
typedef struct linklist
{
	int data;
	linklist* next;
}node, *pnode;

//在头部添加结点
pnode addNodeToHead(pnode head, pnode newnode)
{
	if (head == NULL)
	{
		newnode->next = NULL;
		head = newnode;
		return head;
	}
	newnode->next = head;
	head = newnode;
	return head;
}

//从前往后顺序遍历找到待删除结点算法
pnode deleteNode(pnode head, pnode beDeleted)
{
	if (head == NULL || beDeleted == NULL)
		return NULL;

	pnode temp = head;
	if (temp->data == beDeleted->data)
	{
		head = head->next;
		return head;
	}
	while (temp->next->data != beDeleted->data)
	{
		temp = temp->next;
		if (temp->next == NULL)
			return NULL;
	}
	pnode a = temp->next;
	temp->next = a->next;
	free(a);
	return head;
}
//不需要遍历待删除结点前面的结点，用待删除结点后的结点覆盖待删除结点
pnode DeleteNodeEffect(pnode head, pnode beDelected)
{
	pnode temp = head;
	if (temp == NULL || beDelected == NULL)
		return NULL;
	//链表中只有一个结点
	if (temp->next == NULL)
	{
		delete temp;
		temp = NULL;
	}
	//被删除结点不在尾部
	if (beDelected->next != NULL)
	{
		pnode Dnext = beDelected->next;
		beDelected->data = Dnext->data;
		beDelected->next = Dnext->next;
		delete Dnext;
	}
	//被删除结点在尾部，就需要顺序遍历
	if (beDelected->next == NULL)
	{
		pnode temp = head;
		while (temp->next != beDelected)
		{
			temp = temp->next;
		}
		temp->next = NULL;
		delete beDelected;
	}
	return head;
}

//遍历链表输出
void output(pnode head)
{
	pnode temp = head;
	while (temp != NULL)
	{
		cout << temp->data << '\t';
		temp = temp->next;
	}
}

int main()
{
	int a[] = { 1,2,3,4,5,6,7,8,9 };
	pnode head = NULL;	
	for (int i = 0; i < 9; i++)
	{
		pnode newnode = new node();
		newnode->data = a[i];
		head = addNodeToHead(head, newnode);
	}
	output(head);
        cout<<endl;

	pnode beDeleted = new node();
	beDeleted = head->next->next;
	//head = deleteNode(head, beDeleted);
	head = DeleteNodeEffect(head, beDeleted);
	output(head);
}