Kotlin 函数参数:Val 不能重新赋值 [英] Kotlin function parameter: Val cannot be reassigned

问题描述

我用 Kotlin 编写了红黑树.Fun insertFixup 在插入新元素后恢复平衡(z: Node? 是新元素).树平衡算法取自此处(第 2-3 页).问题是 Kotlin 不允许我将 z 重新分配给 z.parent 和 z.父.父.我希望 z 成为一个 指针.问题是如何让 Kotlin 理解我想从他那里得到什么?

I have written Red–black tree in Kotlin. Fun insertFixup restores balance after inserting new element (z: Node? is new element). Algorithm of tree balancing is taken from here (pages 2-3). The problem is that Kotlin does not allow me to reassign z to z.parent and z.parent.parent. I want z to be a pointer. The question is how to make Kotlin understand what I want from him?

class Node(key: Int) {...}

class BinarySearchTree {
    var root: Node? = null

    fun insert(newNode: Node) {...}

    fun RotateLeft(x: Node?) {...}

    fun RotateRight(x: Node?) {...}

    fun insertFixup(z: Node?) {
        var y: Node?
        while (z?.parent?.color == "RED") {
            if (z?.parent == z?.parent?.parent?.left) {
                y = z?.parent?.parent?.right
                if (y?.color == "RED") {
                    z?.parent?.color = "BLACK"
                    y?.color = "BLACK"
                    z?.parent?.parent?.color = "RED"
                    z = z?.parent?.parent
                }
                if (z == z?.parent?.right) {
                    z = z?.parent
                    RotateLeft(z)
                    z?.parent?.color = "BLACK"
                    z?.parent?.parent?.color = "RED"
                    RotateRight(z?.parent?.parent)
                }
            } else {
                y = z?.parent?.parent?.left
                if (y?.color == "RED") {
                    z?.parent?.color = "BLACK"
                    y?.color = "BLACK"
                    z?.parent?.parent?.color = "RED"
                    z = z?.parent?.parent
                }
                if (z != z?.parent?.left) {
                    z = z?.parent
                    RotateLeft(z)
                    z?.parent?.color = "BLACK"
                    z?.parent?.parent?.color = "RED"
                    RotateRight(z?.parent?.parent)
                }
            }
        }
        root?.color = "BLACK"
    }
}

fun main(args: Array<String>) {
    val bst = BinarySearchTree()

    while (true) {
        var newNode = Node(readLine()!!.toInt())
        bst.insert(newNode)
        bst.insertFixup(newNode)
    }
}

UPD:谢谢大家！所有答案都很有帮助，我在您的回复中找到了解决方案.

UPD: Thanks to all! All the answers were helpful and I have found the solution in your replies.

推荐答案

Kotlin 中的函数参数基本上都是只读的 val 的函数内部，所以 z 在这里将始终引用传入的原始对象.

Function parameters in Kotlin are basically read-only val's inside the function, so z here will always refer to the original object that was passed in.

如果您需要在函数运行时修改它指向的内容，则必须在函数的开头制作它的本地副本，然后您可以将其设为 var.

If you need to modify what it points to while your function is running, you'll have to make a local copy of it at the beginning of the function, and then you can make that a var.

例如，您可以像这样启动您的函数，它可以让您稍后重新分配此本地 var:

For example, you could start your function like this, which lets you reassign this local var later:

fun insertFixup(_z: Node?) {
    var z = _z
    // ...
    z = z.parent
    // ...
}

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