链表（四）——带表头的单向链表

（2）使用带表头的单向链表的优点：不用考虑头结点的单独处理。
表头节点：数据域没有值，指针域指向单向链表中数据域含值的第一个结点。

2.代表头的单向链表的基本操作

#include <stdio.h>
#include <malloc.h>

#define NULL	0

typedef struct node {
	int data;
	struct node *next;
}ElemSN;


ElemSN * creat_link(int ms); //创建一个带表头的链表
void print_link(ElemSN *head); //输出单向链表
void delete_node(ElemSN *head, int x); //删除结点
void insert_node(ElemSN *head, int x); //插入结点
void clear_link(ElemSN *head); //删除链表

int main()
{
	ElemSN *head;
	int ms, x;

	printf("Please input node number:");
	scanf("%d", &ms);
	head = creat_link(ms); //创建链表
	print_link(head);
	printf("Please input delete node:");
	scanf("%d", &x);
	delete_node(head, x); //删除结点
	print_link(head);
	printf("Please input insert node:");
	scanf("%d", &x);
	insert_node(head, x);
	print_link(head);
	clear_link(head);
	print_link(head);
}

ElemSN * creat_link(int ms) //带表头的单向链表除了头结点其余结点
                            //创建方法一致，所以不用逆向创建
{
	ElemSN *h = NULL, *p;
	int i, x;

	h = p = (ElemSN *)malloc(sizeof(ElemSN));
	for(i = 0; i < ms; i++)
	{
		p->next = (ElemSN *)malloc(sizeof(ElemSN)); //创建新结点
		printf("Please input node data:");
		scanf("%d", &x);
		p->next->data = x; //新结点初始化
		p->next->next = NULL; //新结点初始化
		p = p->next;
	}

	return h;
}

void print_link(ElemSN *head)
{
	if(NULL == head->next)
	{
		printf("Link is null.");
	}
	for(head=head->next; head; head = head->next)
	{
		printf("%d ", head->data);
	}
	printf("\n");
}

void delete_node(ElemSN *head, int x)
{
	ElemSN *p, *q;

	for(p=head, q=head->next; q && q->data != x; p=q, q=q->next){}
	if(q != NULL)
	{
		p->next = q->next;
		free(q);
	}
}

void insert_node(ElemSN *head, int x)
{
	ElemSN *p, *q;

	if(NULL == head)
		return;

	if(NULL == head->next) //链表为空时
	{
		head->next = (ElemSN *)malloc(sizeof(ElemSN));
		head->next->next = NULL;
		head->next->data = x;
		return;
	}

	p = head;
	q = head->next;
	if(NULL == q->next) //链表中仅仅有一个元素时
	{
		p->next = (ElemSN *)malloc(sizeof(ElemSN));
		p->next->data = x;
		p->next->next = q;
		return;
	}

	if(q->data >= q->next->data) //递减链表
	{
		for(; q && q->data >= x; p = q, q = q->next){}
	}
	else //递增链表
	{
		for(; q && q->data <= x; p = q, q = q->next){}
	}

	if(NULL == q) //插入在链表尾部
	{
		q = (ElemSN *)malloc(sizeof(ElemSN));
		q->data = x;
		q->next = NULL;
		p->next = q;
	}
	else if(q == head->next) //插入在表首
	{
		q = (ElemSN *)malloc(sizeof(ElemSN));
		q->data = x;
		q->next = head->next;
		head->next = q;
	}
	else //插入在中间位置
	{
		p->next = (ElemSN *)malloc(sizeof(ElemSN));
		p->next->next = q;
		p->next->data = x;
	}
}

void clear_link(ElemSN *head)
{
	ElemSN *p;

	while(head->next)
	{
		p = head->next;
		head->next = p->next;
		free(p);
	}
}