这是道简单题，直接用快慢指针，代码如下

public class Solution {
    public boolean hasCycle(ListNode head) {
        if (head == null || head.next == null) {
            return false;
        }
        ListNode slow = head;
        ListNode fast = head.next;
        while (slow != fast) {
            //遇到null则return false
            if (fast == null || fast.next == null) {
                return false;
            }
            //慢指针每次走一步
            slow = slow.next;
            //快指针每次走两步
            fast = fast.next.next;
        }
        return true;
    }
}

题目链接：题单 - 力扣（LeetCode）全球极客挚爱的技术成长平台