题目：

题解：

public class Solution {public ListNode detectCycle(ListNode head) {if (head == null) {return null;}ListNode slow = head, fast = head;while (fast != null) {slow = slow.next;if (fast.next != null) {fast = fast.next.next;} else {return null;}if (fast == slow) {ListNode ptr = head;while (ptr != slow) {ptr = ptr.next;slow = slow.next;}return ptr;}}return null;}
}