参考回答：
是否有环：
while (faster.next != null && faster.next.next != null) { faster = faster.next.next; slower = slower.next; if (faster == slower) { return true; }}return false;判断环入口：
Node meet = null;while (faster.next != null && faster.next.next != null) {faster = faster.next.next;slower = slower.next;if (faster == slower) {meet = faster;}}if (meet != null) {newSlower = head;while (newSlower != slower) {newSlower = newSlower.next;slower = slower.next;}return newSlower;}return null;
