设置一个fast指针，一个slow指针，fast一次走两步，slow一次走一步。如果fast和slow相遇，则说明有环。反之没相遇则无环。

注意快慢指针的while循环条件是fast.next != null && fast.next.next != null

/*** 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) return false;ListNode fast = head, slow = head;while(fast.next!=null && fast.next.next!=null){fast = fast.next.next;slow = slow.next;if(fast == slow) return true;}return false;}
}