如何在JavaScript中将自下而上的递归算法转换为迭代堆栈 [英] How to convert bottom-up recursive algorithm to iterative stack in JavaScript
问题描述
给出以下算法:
console.log(JSON.stringify(create(0), null, 2))
function create(i) {
if (i == 5) return
return new Klass(i, create(i + 1), create(i + 1))
}
function Klass(i, l, r) {
this.i = i
this.l = l
this.r = r
}
它在 create(0)
last >中创建 Klass
em>,在创建所有子项之后,递归地。因此它首先创建叶节点,然后将其传递给父节点等。
It creates the Klass
in create(0)
last, after creating all the children, recursively. So it creates the leaf nodes first, then passes that up to the parent, etc.
想知道如何使用没有递归的堆栈来执行此操作。让我的头受伤:)我理解如何使用堆栈从上到下创建,但不是自下而上。对于自上而下,它基本上是这样的:
Wondering how to do this using a stack without recursion. Making my head hurt :). I understand how to use a stack to create from the top-down, but not the bottom up. For top-down, it's essentially this:
var stack = [0]
while (stack.length) {
var i = stack.pop()
// do work
stack.push(children)
}
从下到上,我看不出它应该如何运作。这就是我陷入困境的地方:
From the bottom up, I can't see how it should work. This is where I get stuck:
function create(i) {
var stack = []
stack.push([i, 'open'])
stack.push([i, 'close'])
while (stack.length) {
var node = stack.pop()
if (node[1] == 'open') {
stack.push([ node[0] + 1, 'open' ])
stack.push([ node[0] + 1, 'close' ])
} else {
// ?? not sure how to get to this point
var klass = new Klass(node[0], node[2], node[3])
// ??
}
}
}
推荐答案
将任何递归代码机械转换为堆栈机器并非易事。自动状态转换会产生非常复杂的代码,只需考虑C#-s或BabelJS-s生成器。但可以肯定的是,它可以完成,但是你需要可变的堆栈帧和/或寄存器。让我们看看我们面临的问题:
It's not trivial to mechanically transform any recursive code into a stack machine. Automatic stateful transformations produce very complex code, just think of C#-s or BabelJS-s generators. But sure, it can be done, but you will need mutable stackframes and/or registers. Let's see the problems we are facing:
我们必须在堆栈本身上存储一些状态变量/指令指针。这是您使用open
和close
标记进行模拟。
We have to store some state variable/instruction pointer on the stack itself. This is what you are emulating with the "open"
and "close"
markers.
有很多方法:
- 将其存储在临时寄存器中
- 将函数引用传递给字段((对象,字段名)对),模拟
out
参数 - 使用像@CtheSky这样的第二个堆栈
- storing it in a temporary register
- passing the function a reference to a field (an (object, fieldname) pair), emulating
out
parameters - using a second stack like @CtheSky did
使用可变堆栈帧和结果寄存器,转换后的代码看起来像这样:
Using mutable stack frames and a result register the transformed code would look something like this:
console.log(JSON.stringify(create(0), null, 2))
function Klass(i, l, r) {
this.i = i
this.l = l
this.r = r
}
function Frame(i) {
this.ip = 0;
this.i = i;
this.left = null;
}
function create(i) {
var result;
var stack = [new Frame(i)];
while (stack.length > 0) {
var frame = stack[stack.length - 1];
switch (frame.ip) {
case 0:
if (frame.i === 5) {
result = undefined;
stack.pop();
break;
}
stack.push(new Frame(frame.i + 1));
frame.ip = 1;
break;
case 1:
frame.left = result;
stack.push(new Frame(frame.i + 1));
frame.ip = 2;
break;
case 2:
result = new Klass(frame.i, frame.left, result);
stack.pop();
break;
}
}
return result;
}
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