//找到环的入口    slow=this.head;  slow=slow.next;   fast=fast.next;
public ListNode detectCycle() {ListNode fast = this.head;ListNode slow = this.head;while (fast != null && fast.next != null) {fast = fast.next.next;slow = slow.next;if (slow == fast) {break;}}if (fast == null || fast.next == null) {//没有环return null;}slow = this.head;while (slow != fast) {slow = slow.next;fast = fast.next;}return fast;
}