求环的入口结点

/*
struct ListNode {
int val;
struct ListNode *next;
ListNode(int x) :
val(x), next(NULL) {
}
};
*/
class Solution {
public:
ListNode* MeetingNode(ListNode* pHead)
{  //找到环的入口结点
if(pHead==NULL)
return pHead;
ListNode *s=pHead->next,*f=NULL;
if(s!=NULL)
f=s->next;
while(s!=NULL && f!=NULL)
{
if(s==f)
return f;
s=s->next;
f=f->next;
if(f!=NULL)
f=f->next;
}
return NULL;
}
ListNode* EntryNodeOfLoop(ListNode* pHead)
{
if(pHead==NULL)
return pHead;
ListNode* meetNode=MeetingNode(pHead);
if(meetNode==NULL)
return NULL;
ListNode* p=pHead;
while(p!=meetNode)
{
p=p->next;
meetNode=meetNode->next;
}
return p;
}
};