// 不能使用额外的存储空间

     public boolean hasCycle(ListNode head) {

         // 如果存在环的 两个指针用不一样的速度 会相遇

         ListNode fastNode = head;

         ListNode slowNode = head;

         while (fastNode != null && fastNode.next != null) {

             fastNode = fastNode.next.next;

             slowNode = slowNode.next;

             if (fastNode == slowNode) {

                 return true;

             }

         }

         return false;

     }

 // 如果有环，相遇的时候与起点相差的步数等于从head到起点的步数

     public ListNode detectCycle(ListNode head) {

         ListNode pre = head;

         ListNode slow = head;

         ListNode fast = head;

         while (fast != null && fast.next != null) {

             fast = fast.next.next;

             slow = slow.next;

             if (slow == fast) {

                 while (pre != slow) {

                     slow = slow.next;

                     pre = pre.next;

                 }

                 return pre;

             }

         }

         return null;

     }