java 快速排序 递归_Java递归快速入门

java 快速排序 递归

The function calls itself until someone stops it.

该函数将自行调用，直到有人停止它为止。

Recursion can feel difficult to new developers. Perhaps that's because many resources teach it using algorithmic examples (Fibonacci, linked-lists). This piece will hopefully introduce things plainly, using one simple example.

对于新开发人员而言，递归可能会感到困难。 也许是因为许多资源都使用算法示例(斐波纳契(Fibonacci)，链接列表)来教它。 希望通过一个简单的例子，这篇文章将简单地介绍一下。

核心思想 (Core Idea)

Recursion is when a function calls itself until someone stops it. If no one stops it then it'll recurse (call itself) forever.

递归是指函数调用自身直到有人停止它。 如果没有人停止，它将永远递归 (自称)。

Recursive functions let you perform a unit of work multiple times. This is exactly what for/while loops let us accomplish! Sometimes, however, recursive solutions are a more elegant approach to solving a problem.

递归函数使您可以多次执行一个工作单元。 这正是for/while循环让我们完成的！ 但是，有时递归解决方案是解决问题的一种更优雅的方法。

倒数功能 (Countdown Function)

Let's create a function that counts down from a given number. We'll use it like this.

让我们创建一个从给定数字开始倒数的函数。 我们将像这样使用它。

countDownFrom(5);
// 5
// 4
// 3
// 2
// 1

And here's our algorithm to solve this problem.

这是我们解决此问题的算法。

1. Take one parameter called number. This is our starting point.

   取一个称为number参数。 这是我们的出发点。

2. Go from number down to 0, logging each one along the way.

   从number降低到0 ，并沿途记录每个number 。

We'll start with a for loop approach and then compare it to a recursive one.

我们将从for循环方法开始，然后将其与递归方法进行比较。

命令式方法(循环) (Imperative approach (loops))

function countDownFrom(number) {
	for (let i = number; i > 0; i--) {
		console.log(i);
	}	
}

countDownFrom(5);
// 5
// 4
// 3
// 2
// 1

This one contains both algorithmic steps.

这包含两个算法步骤。

1. ✅ Take one parameter called number.

   ✅取一个称为number参数。

2. ✅ Log everything from number to 0.

   ✅记录从number到0所有内容。

递归方法 (Recursive approach)

function countDownFrom(number) {
	if (number === 0) {
		return;
	}

    console.log(number);    
    countDownFrom(number - 1);
}

countDownFrom(5);
// 5
// 4
// 3
// 2
// 1

This one also passes.

这也过去了。

1. ✅ Take one parameter called number.

   ✅取一个称为number参数。

2. ✅ Log everything from number to 0.

   ✅记录从number到0所有内容。

So conceptually the two approaches are the same. However, they get the job done in different ways.

因此，从概念上讲，这两种方法是相同的。 但是，他们以不同的方式完成工作。

调试我们的命令性解决方案 (Debugging our imperative solution)

For a more visual example, let's put a debugger in our loop version and throw it into Chrome Developer Tools.

对于更直观的示例，让我们在循环版本中放入debugger ，然后将其放入Chrome开发者工具中。

function countDownFrom(number) {
	for (let i = number; i > 0; i--) {
		console.log(i);
		debugger;
	}	
}

See how it uses an extra variable, i, to track the current number? As you iterate i decreases, eventually hitting 0 and terminating.

看看它如何使用额外的变量i来跟踪当前数字吗？ 随着您的迭代， i逐渐减少，最终达到0并终止。

And in the for loop we specified "stop if i > 0".

在for循环中，我们指定“如果i > 0停止”。

调试我们的递归解决方案 (Debugging our recursive solution)

function countDownFrom(number) {
	if (number === 0) {
		return;
	}

    console.log(number);
	
	debugger;

    countDownFrom(number - 1);
}

The recursive version doesn't need extra variables to track its progress. Notice how the pile of functions (call stack) grows as we recurse?

递归版本不需要额外的变量来跟踪其进度。 注意我们递归时函数堆( 调用堆栈 )如何增长？

That's because each call to countDownFrom adds to the stack, feeding it number - 1. By doing this we're we're passing along an updated number each time. No extra state needed!

这是因为对countDownFrom的每次调用countDownFrom添加到堆栈中，并向其提供number - 1 。 通过这样做，我们每次都传递一个更新的number 。 不需要额外的状态！

That's main difference between the two approaches.

这是两种方法之间的主要区别。

1. Iterative uses internal state (extra variables for counting, etc).
   迭代使用内部状态(用于计数的其他变量等)。
2. Recursive does not, it simply passes updated parameters between each call.
   递归没有，它只是在每次调用之间传递更新的参数。

But how does either version know when to stop?

但是，哪个版本知道何时停止？

无限循环 (Infinite Loops)

In your travels, you may have been warned about the dreaded infinite loop.

在旅行中，您可能已经被警告过可怕的无限循环。

 THIS RUNS FOREVER, BE WARNED 
while (true) { console.log('WHY DID YOU RUN THIS?!' }

 THIS RUNS FOREVER, BE WARNED 
for (i = 0;;) { console.log('WHY DID YOU RUN THIS?!') }

Since they'd theoretically run forever, an infinite loop will halt your program and possibly crash your browser. You can prevent them by always coding a stopping condition.

由于理论上它们将永远运行，因此无限循环将暂停您的程序，并可能导致浏览器崩溃。 您可以通过始终编写停止条件来防止它们发生。

✅ This does not run forever
x = 0;
while (x < 3) { console.log(x); x++; }

✅ This does not run forever
for (x = 0; x < 3; x++) { console.log(x); }

In both cases we log x, increment it, and stop when it becomes 3. Our countDownFrom function had similar logic.

在这两种情况下，我们都记录x ，递增x ，然后在x变为3时停止。 我们的countDownFrom函数具有类似的逻辑。

// Stop at 0
for (let i = number; i > 0; i--)

Again, loops need extra state to determine when they should stop. That's what x and i are for.

同样，循环需要额外的状态来确定何时停止。 这就是x和i是。

无限递归 (Infinite Recursion)

Recursion also presents the same danger. It's not hard to write a self-referencing function that'll crash your browser.

递归也存在同样的危险。 编写会导致浏览器崩溃的自引用功能并不难。

THIS RUNS FOREVER, BE WARNED
function run() {
    console.log('running');
    run();
}

run();
// running
// running
// ...

Without a stopping condition, run will forever call itself. You can fix that with an if statement.

在没有停止条件的情况下， run将永远自我调用。 您可以使用if语句解决该问题。

✅ This does not run forever

function run(x) {
    if (x === 3) return;
    
    console.log('running');
    run(x + 1);
}

run(0);
// running
// running
// running

// x is 3 now, we're done.

基本情况 (Base case)

This is known as the base case–our recursive countDownFrom had one.

这被称为基本案例–我们的递归countDownFrom有一个。

if (number === 0) {
    return;
}

It's the same idea as our loop's stopping logic. Whichever approach you pick, always remember that at some point it needs to be stopped.

这与循环的停止逻辑相同。 无论选择哪种方法，请始终记住，必须在某个时候停止它 。

摘要 (Summary)

• Recursion is when a function calls itself until someone stops it.
  递归是一个函数调用自身直到有人停止它的时间。
• It can be used instead of a loop.
  可以使用它代替循环。
• If no one stops it, it'll recurse forever and crash your program.
  如果没有人停止它，它将永远递归并崩溃您的程序。

• A base case is a condition that stops the recursion. Don't forget to add them!

  基本情况是停止递归的条件。 不要忘记添加它们！

• Loops use extra state variables for tracking and counting, while recursion only uses the provided parameters.
  循环使用额外的状态变量进行跟踪和计数，而递归仅使用提供的参数。

谢谢阅读 (Thanks for reading)

For more content like this, check out https://yazeedb.com. And please let me know what else you'd like to see! My DMs are open on Twitter.

有关此类的更多内容，请访问https://yazeedb.com 。 并且，请让我知道您还想看到什么！ 我的DM在Twitter上打开。

Until next time!

直到下一次！

翻译自: https://www.freecodecamp.org/news/quick-intro-to-recursion/

java 快速排序 递归