题解 | 链表中环的入口结点

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;
        }
        HashMap<ListNode, Boolean> exist = new HashMap<>();
        ListNode current = pHead;
        while(current != null) {
            if (exist.get(current) != null) {
                return current;
            }
            exist.put(current, true);
            current = current.next;
        }
        return null;
    }
}

判断链表里的环还是比较简单的，借助map来判断之前的节点是否存在过，然后遍历一遍整个链表就完成了