哈希表法

import java.util.*;
/*
 public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}
*/
public class Solution {

    public ListNode EntryNodeOfLoop(ListNode pHead) {
        
       Set<ListNode> set=new HashSet<>();
       boolean flag=false;
        while(pHead!=null){
           if(!set.add(pHead)){
            flag=true;
            break;
           }
           pHead=pHead.next;
        }

        return pHead;
    }
}

首次重复经过的即环的入口点

快慢指针

图解

import java.util.*;
/*
 public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}
*/
public class Solution {

    public ListNode EntryNodeOfLoop(ListNode pHead) {
      if(pHead==null)return null;
      ListNode fast=pHead;
      ListNode slow=pHead;
      boolean flag=false;
        while(fast!=null&&fast.next!=null){
           fast=fast.next.next;
           slow=slow.next;
           if(fast==slow){
            flag=true;
            break;}
        }
        //有环
        if(flag==true){
           slow=pHead;
           while(true){
            if(fast==slow)break;
            fast=fast.next;
            slow=slow.next;
           }
           return slow;
        }
        return null;
    }
}