Swift: 这如何创建一个二叉树？

Question

所以我正在按照在线教程学习，除了这部分我什么都懂。该人向我解释说，这会创建一个二叉树，可以说一个项目链接到两个项目。在这种情况下，一个页面链接到两个页面。我不明白在这个例子中这是如何工作的，其中 struct Adventure 创建了二叉树。任何帮助将不胜感激。目前，我为完全不了解这一点感到非常难过。

import Foundation

class Page {
    let story: Story

    typealias Choice = (title: String, page:Page)

    var firstChoice: Choice?
    var secondChoice: Choice? 

    init(story: Story) {
        self.story = story
    }
}

    extension Page {
// adds the page
        func addChoiceWith(title: String, story:Story) -> Page {
            let page = Page(story:story)
            return addChoiceWith(title: title, page: page)
        }

// creates branches
        func addChoiceWith(title: String, page: Page) -> Page {
            switch (firstChoice, secondChoice) {
            case (.some, .some) : return self
            case (.none, .none), (.none, .some): firstChoice = (title, page)
            case (.some, .none): secondChoice = (title,page)

            }

            return page
        }
    }


    struct Adventure {
        static var story: Page {
            let returnTrip = Page(story: .returnTrip)
            let touchdown = returnTrip.addChoiceWith(title: "Stop and Investigate", story: .touchDown)
            let homeward = returnTrip.addChoiceWith(title: "Continue home to Earth", story: .homeward)
            let rover = touchdown.addChoiceWith(title: "Explore the Rover", story: .rover)
            let crate = touchdown.addChoiceWith(title: "Open the Crate", story: .crate)

            homeward.addChoiceWith(title: "Head back to Mars", page: touchdown)
            let home = homeward.addChoiceWith(title: "Continue Home to Earth", story: .home)

            let cave = rover.addChoiceWith(title: "Explore the Coordinates", story: .cave)
            rover.addChoiceWith(title: "Return to Earth", page: home)

            cave.addChoiceWith(title: "Continue towards faint light", story: .droid)
            cave.addChoiceWith(title: "Refill the ship and explore the rover", page: rover)

            crate.addChoiceWith(title: "Explore the Rover", page: rover)
            crate.addChoiceWith(title: "Use the key", story: .monster)

            return returnTrip
        }
    }

Answer 1

每个Page代表树中的一个节点，每个Choice代表分支。一个Story就像节点的内容。 firstChoice.page和secondChoice.page表示一个节点的两个children。

Adventure.story 属性创建树和 returns 根节点。您可以使用根访问所有其他节点：

Adventure.story.firstChoice.page.secondChoice.page

现在，让我们看看树是如何构建的。请注意，这不是您一直看到的标准二叉树。一些节点的 parents 为 children。但是每个节点确实有两个children.

我会把代码变成伪代码。你可以按照这个代码，在一张纸上，自己画出这棵树。

Create a node called "returnTrip"
Add a child to returnTrip called "touchDown"
Add a child to returnTrip called "homeward"
Add a child to touchDown called "rover"
Add a child to touchDown called "crate"
Connect homeward back to touchDown
Add a child to homeward called "home"
Add a child to rover called "cave"
Connect rover to home
Add a child to cave called "droid"
Connect cave to rover
Connect crate to rover
Add a child to crate called "monster"

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