链表中环的入口节点

//一个链表中包含环，请找出该链表的环的入口结点。class Solution {public:    ListNode* EntryNodeOfLoop(ListNode* pHead)    {        ListNode *slow = pHead;         ListNode *fast = pHead;        do{            if(fast == NULL || fast->next==NULL)                return NULL;            fast = fast->next->next;            slow = slow->next;            }while(slow != fast);        slow = pHead;        while(slow != fast){            slow = slow->next;            fast = fast->next;        }        return slow;        }};