利用快慢指针查看链表是否有闭环

创建两个指针分别指向头节点，快指针每次移动两位，慢指针每次移动一位，如果快慢指针能相遇则说明链表有闭环。

/// 快慢指针解决链表是否有闭环
    private func testCircle() {
        let firstNode = Node(item: "1", next: nil)
        let secondNode = Node(item: "2", next: nil)
        let thirdNode = Node(item: "3", next: nil)
        let forthNode = Node(item: "4", next: nil)
        let fifthNode = Node(item: "5", next: nil)
        let sixthNode = Node(item: "6", next: nil)
        let seventhNode = Node(item: "7", next: nil)
        let eightNode = Node(item: "8", next: nil)
        
        firstNode.next = secondNode
        secondNode.next = thirdNode
        thirdNode.next = forthNode
        forthNode.next = fifthNode
        fifthNode.next = sixthNode
        sixthNode.next = seventhNode
        seventhNode.next = eightNode
        
        eightNode.next = fifthNode
        
        let isCir = isCircle(firstNode: firstNode)
        print("isCircle:\(isCir)")
    }
    
    private func isCircle(firstNode: Node) -> Bool {
        var fast = firstNode
        var slow = firstNode
        while fast.next?.next != nil {
            fast = fast.next!.next!
            slow = slow.next!
            if fast.item == slow.item { // 应该是比较节点是否相同
                return true
            }
        }
        return false
    }

demo地址：https://github.com/yangguanghei/studyDateStructure