题目：

题解：

func detectCycle(head *ListNode) *ListNode {
    slow, fast := head, head
    for fast != nil {
        slow = slow.Next
        if fast.Next == nil {
            return nil
        }
        fast = fast.Next.Next
        if fast == slow {
            p := head
            for p != slow {
                p = p.Next
                slow = slow.Next
            }
            return p
        }
    }
    return nil
}