先判断是否有环， 再用entry找那个Node是起点

public ListNode DetectNode(ListNode head){
  if(head ==null) return null;
  ListNode slow = head;
  ListNode fast = head;
  ListNode entry = head;

while(fast.next!=null && fast.next.next!=null){
   fast = fast.next.next;
   slow  =slow.next;
 if(fast == slow){
   while(entry!=slow){
     entry = entry.next;
     slow = slow.next;
  }
 return entry;
}
}
return null;
}


class ListNode{
  int val;
  ListNode next;
  ListNode(int x){
    val = x;
    next  =null;
  }
}