链表中环的入口结点

public ListNode EntryNodeOfLoop(ListNode pHead) {
        if(pHead==null) return null;
        ListNode slow = pHead,fast = pHead;
        while(fast!=null&&fast.next!=null){
            fast = fast.next.next;
            slow = slow.next;
            if(slow==fast){
                fast = pHead;
                while(fast!=slow){
                    slow  = slow.next;
                    fast = fast.next;
                }
                if(fast==slow) return slow;
            }
        }
        return null;
        
    }