swift创建链表二

检查链表是否为循环列表

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