public ListNode EntryNodeOfLoop(ListNode pHead)
    {
        ListNode fast=pHead;
        ListNode slow=pHead;
         
        ListNode cur=pHead;
        while(cur!=null){
            if (fast.next==null||fast.next.next==null) {
                return null;
            }
            fast=fast.next.next;
            slow=slow.next;
            if(slow==fast){
                break;
            }
            cur=cur.next;
        }
         
        fast=pHead;
        while(fast!=slow){
            fast=fast.next;
            slow=slow.next;
        }
        return slow;
    }