环形链表

func hasCycle(_ head: ListNode?) -> Bool {
    var fast = head, slow = head
    while fast?.next != nil && fast?.next?.next != nil {
        fast = fast?.next?.next
        slow = slow?.next
        if slow === fast {
            return true
        }
    }
    return false
}

func detectCycle(_ head: ListNode?) -> ListNode? {
    var fast = head, slow = head
    while fast != nil {
        slow = slow?.next
        fast = fast?.next?.next
        if slow === fast {
            slow = head
            while slow !== fast {
                slow = slow?.next
                fast = fast?.next
            }
            return slow
        }
    }
    return nil
}