/*

 struct ListNode {

     int val;

     struct ListNode *next;

     ListNode(int x) :

         val(x), next(NULL) {

     }

 };

 */

 class Solution {

 public:

     ListNode* EntryNodeOfLoop(ListNode* pHead)

     {

         if(pHead == NULL)

             return NULL;

         set<ListNode*> Set;

         while(pHead){

             if(!Set.insert(pHead).second)

                 return pHead;

             pHead = pHead->next;

         }

         return NULL;

     }

 };