142. 环形链表 II

public class Num142环形链表II {
    public ListNode detectCycle(ListNode head) {
        ListNode slow = head, fast = head;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            if (fast == slow) {
                slow = head;
                while (fast != slow) {
                    fast = fast.next;
                    slow = slow.next;
                }
                return slow;
            }
        }
        return null;
    }
}