解题思路：

使用set判断是否重复添加，如果set加入不进去证明之前到达过该节点，有环

public class Solution {public ListNode detectCycle(ListNode head) {Set<ListNode> set = new HashSet<>();ListNode cur = head;while (cur != null) {boolean tag = set.add(cur);if (tag == false) return cur;cur = cur.next;}return null;}
}