//基於hash索引
    public ListNode EntryNodeOfLoop1(ListNode pHead)
    {   
        ListNode next = pHead;
        Set<ListNode> set = new HashSet<ListNode>();
        ListNode result = null;//可能不存在環
        while(next!=null){
         if(!set.contains(next)){
          set.add(next);
          next = next.next;
         }else{
          result = next;
          break;
         }
        }
        return result;
    }
    //基於快慢指標
    public static ListNode EntryNodeOfLoop2(ListNode pHead){
     //找到環的大小
     int m= 0;
     ListNode next = pHead.next;
     ListNode fast =pHead;
     ListNode low = pHead;
     while(next!=null&&next.next!=null){
      low = low.next;
      fast =next.next;
      next = fast.next;
      m++;
      if(low == fast)
       break;                  
     }
     if(next==null||next.next==null)//說明沒有環
      return null;
     //找到連結串列中倒數第m個節點
     //快指標先走m步
     fast = pHead;
     low = pHead;
     for(int  i = 0 ;i<m; i++){
      fast = fast.next;
     }
     while(fast != low){
      fast = fast.next;
      low = low.next;
     }
     return fast;
    }