class ListNode {

       int val;

       ListNode next;

       ListNode(int x) {

            val = x;

            next = null;

     }

 }

public class test1 {

    public boolean hasCycle(ListNode head) {

        if(head==null || head.next==null){

            //头指针为空或者只有头节点，无环

            return false;

        }

        ListNode slow,fast = new ListNode(0);

        slow = head.next;

        fast = head.next.next;

        while(true){

            if(fast==null||fast.next==null){

                //fast走到链表尾

                return false;

            }else if(fast.next==slow || fast==slow){

                return true;

            }else{

                slow = slow.next;// slow每次走一步

                fast = fast.next.next;//fast每次走两步

            }

        }

    }

    public static void main(String[] args) {

        ListNode node1 = new ListNode(1),node2 = new ListNode(2),node3 = new ListNode(3),node4=new ListNode(4);

        node1.next=node2;

        node2.next=node3;

        node3.next=node4;

        node4.next=node1;

        test1 test = new test1();

        System.out.println(test.hasCycle(node1));

    }

}