C语言：循环单链表的创建，遍历，插入

/*
1.循环单链表的创建，由尾节点指向该链表，
2.循环链表遍历
3.求链表的长度
4.向链表中插入元素
*/

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

typedef struct node
{
int data;
struct node * pNext;
}NODE, * PNODE;

PNODE creat_list();    //创建链表
void travese(PNODE T);   //遍历链表
void insert(PNODE &T, int pos, int val);  //插入元素
int length(PNODE T);  //求链表的长度

int main(void)
{
//返回循环链表的尾指针
PNODE T = creat_list();
travese(T);

//若在第一个有效节点前插入新节点5
insert(T, 4, 5);
travese(T);

return 0;
}

//循环单链表的创建，返回尾节点
PNODE creat_list()
{
PNODE pHead = (PNODE)malloc(sizeof(NODE));  //头结点
pHead->pNext = pHead;

PNODE pTail = pHead; //尾指针
PNODE pNew;

int len;
int i;
int val;

printf("输入此循环链表的长度 len = ");
scanf("%d", &len);

for(i = 0; i < len; i++)
{
printf("输入第%d个元素的值 val = ", i+1);
scanf("%d", &val);

pNew = (PNODE)malloc(sizeof(NODE));  //开辟新节点

pNew->data = val;
pNew->pNext = pHead;  //新节点的指针域始终指向头结点

pTail->pNext = pNew;
pTail = pNew;
}

return pTail;  //返回尾指针
}

//循环单链表的遍历
void travese(PNODE T)
{
PNODE pHead = T->pNext;   //pHead指向头结点
PNODE p = pHead->pNext;   //p指向首节点，第一个有效节点

while(p != pHead)  //循环链表的终止条件:（p != 头指针）
{
printf("%d ", p->data);
p = p->pNext;
}
printf("\n");
}

//向第pos个有效节点前，插入新节点
void insert(PNODE &T, int pos, int val)  //可能会改变尾指针的位置
{
PNODE pHead = T->pNext; //pHead指向头结点

PNODE pNew = (PNODE)malloc(sizeof(NODE));
pNew->data = val;

PNODE p = pHead->pNext;  //指向第一个有效节点

int len = length(T);
int count = 0;

if(1 == pos)  //插入到第一个有效节点前
{
pNew->pNext = pHead->pNext;

pHead->pNext = pNew;
}
if(pos >1 && pos < len + 1)
{
while(p != NULL)
{
count ++;
if(count == pos -1)  //pos位置前一个节点
{
pNew->pNext = p->pNext;
p->pNext = pNew;
break;
}
p = p->pNext;
}

}
if(pos == len + 1)  //在链表尾插入一节点
{
pNew->pNext = T->pNext;
T->pNext = pNew;
T = pNew;
}
if(pos > len + 1)
{
printf("插入位置错误！");
return;
}

}

//求链表的长度
int length(PNODE T)
{
PNODE pHead = T->pNext;
PNODE p = pHead->pNext;

int len = 0;

while(p != pHead)
{
len ++;
p = p->pNext;
}
return len;
}