力扣HOT100 - 142. 环形链表 II

解题思路：

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