使用快慢指針就好啦

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public boolean hasCycle(ListNode head) {
        if(head==null||head.next==null) return false;
        ListNode fast = head.next ;
        ListNode slow = head ;
        while(fast!=slow)
        {
            if(fast!=null&&fast.next!=null)
                fast=fast.next.next;
            else 
                return false;
            if(slow!=null)
                slow=slow.next;
            else
                return false;
        }
        return true;
    }
}