链表中环的入口节点

 1 public class Solution {
 2     public ListNode EntryNodeOfLoop(ListNode pHead)
 3     {
 4          if(pHead==null){return null;}
 5          HashSet<ListNode> set=new HashSet<ListNode>();
 6          while(pHead!=null){
 7              if(!set.add(pHead)){
 8                  return pHead;
 9              }
10              pHead=pHead.next;
11          }
12         return null;
13     }
14 }