判断单链表是否带环？若带环，求环的长度？求环的入口点？（C语言）

PSListNode HasCycle(PSListNode pHead)
{
if ((NULL == pHead) || (NULL == pHead->pNextNode))
{
return NULL;
}
else
{
PSListNode pFast = pHead->pNextNode->pNextNode;
PSListNode pSlow = pHead->pNextNode;
//利用快慢指针，让快指针每次走两步，慢指针每次走一步，要是快指针没有走到NULL，且快指针与慢指针指向相同就说明是有环
while (pFast != pSlow)
{
//快指针要是没有指向为空，那么慢指针就不可能指向空（快指针走得快）
if (NULL == pFast)
{
return;
}
else
{
pFast = pFast->pNextNode;
pSlow = pSlow->pNextNode;
if (NULL == pFast)
{
return;
}
else
{
pFast = pFast->pNextNode;
}
}
}
return pFast;
}
}

int GetCyleLen(PSListNode pMeetNode)
{
//默认传的参数是HasCycle函数返回的环中的一个结点
if (NULL == pMeetNode)
{
return 0;
}
else
{
int nCount = 1;
PSListNode pNode = pMeetNode;
while (pMeetNode != pNode->pNextNode)
{
pNode = pNode->pNextNode;
nCount++;
}
return nCount;
}
}
//pMeetNode参数是用HasCycle函数求链表是否有环时pFast指针与pSlow指针的碰撞点
//定律：在链表头，pFast指针与pSlow指针的碰撞点分别设定一个指针，每次各走一步，两个指针必定相遇，则相遇第一点为环入口点
PSListNode FindEnterNode(PSListNode pHead, PSListNode pMeetNode)
{
PSListNode pNode = pHead;
if ((NULL == pHead) || (NULL == pMeetNode))
{
return NULL;
}
while (pNode != pMeetNode)
{
pNode = pNode->pNextNode;
pMeetNode = pMeetNode->pNextNode;
}
return pNode;
}