ARTS-weekly-31

输入：head = [3,2,0,-4,2], pos = 1
输出：tail connects to node index 1
解释：链表中有一个环，其尾部连接到第二个节点。

class Sulution {
    public ListNode detectCycle(ListNode head) {
       if (head == null || head.next == null) {
            return null;
        }
        ListNode fast = head;
        ListNode slow = head;
        boolean hasCycle = false;
        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
            if (fast == slow) {
                hasCycle = true;
                break;
            }
        }
        if (!hasCycle) {
            return null;
        }
        slow = head;
        while (fast != slow) {
            fast = fast.next;
            slow = slow.next;
        }
        return slow;
    }
}