面试题：环形链表的入口节点

public class Solution {
    ListNode EntryNodeOfLoop(ListNode pHead){
        if(pHead==null||pHead.next==null) return null;
        ListNode p1=pHead;
        ListNode p2=pHead;
        while(p1!=null||p1.next!=null){
            p1=p1.next;
            p2=p2.next.next;
            if(p1==p2){
                p1=pHead;
                while(p1!=p2){
                    p1=p1.next;
                    p2=p2.next;
                }
                if(p1==p2)  return p1;
            }
        }
        return null;
    }
}

import java.util.HashSet;
public class Solution {
    public ListNode EntryNodeOfLoop(ListNode pHead){
        HashSet<ListNode> set=new HashSet<ListNode>();
        while(pHead!=null){
            //添加重复值返回false
            if(!set.add(pHead))  return pHead;
            pHead=pHead.next;
        }
        return null;
    }
}