//查找环的入口点

public class Solution {

public ListNode EntryNodeOfLoop(ListNode pHead)

{

if(pHead==null||pHead.next==null)

return null;

ListNode fast = new ListNode(0);

ListNode slow = new ListNode(0);

//

slow=pHead.next;

fast=pHead.next.next;

while(fast!=slow)

{

slow=slow.next;

fast=fast.next.next;

}

fast=pHead;

while(fast!=null)

{

if(fast==slow)

return fast;

slow=slow.next;

fast=fast.next;

}

return slow;

}

}

/*

c++实现

class Solution {

public:

ListNode* EntryNodeOfLoop(ListNode* pHead)

{

if(pHead==NULL || pHead->next==NULL)

return NULL;

set<ListNode*> listSet;

while(pHead!=NULL){

//说明没有找到pHead

if(listSet.find(pHead)==listSet.end()){

listSet.insert(pHead);

pHead=pHead->next;

}

else

return pHead;

}

return NULL;

}

};

*/