vanbrabantf

<?php

  $right = $this->invert($tree->getLeft());
  $left = $this->invert($tree->getRight());

  return new Node($tree->getValue(), $left, $right);

vanbrabantf

<?php 

if ($tree === null) {
        return null;
}

vanbrabantf

<?php

public function invert(?Node $tree): ?Node
{
    if ($tree === null) {
        return null;
    }

    $right = $this->invert($tree->getLeft());
    $left = $this->invert($tree->getRight());

vanbrabantf

<?php

public function invert(Node $tree): Node
{
    $right = $tree->getLeft();
    $left = $tree->getRight();

    return new Node($tree->getValue(), $left, $right);
}

vanbrabantf

<?php

$level3al = new Node('deepAL', null, null);
$level3ar = new Node('deepAR', null, null);

$level3bl = new Node('deepBL', null, null);
$level3br = new Node('deepBR', null, null);

$level2a = new Node('midA', $level3al, $level3ar);
$level2b = new Node('midB', $level3bl, $level3br);

vanbrabantf

<?php

class Node
{
    private $value;
    private $left;
    private $right;

    public function __construct(string $value, ?Node $left, ?Node $right)
    {

vanbrabantf

<?php
declare(strict_types=1);

final class CustomerName
{
    private $firstName;

    private $lastName;

    public function __construct(string $firstName, string $lastName)

vanbrabantf

<?php
    public function setFirstName(string $firstName): CustomerName
    {
        return new self($firstName, $this->lastName());
    }

vanbrabantf

<?php
    public function __construct(string $firstName, string $lastName)
    {
        Assert::stringNotEmpty($firstName);
        Assert::stringNotEmpty($lastName);
        Assert::alpha($firstName);
        Assert::alpha($lastName);

        $this->firstName = $firstName;
        $this->lastName = $lastName;

vanbrabantf

<?php
    public function sendMail(
        CustomerName $customerName,
        Email $email
    ): void
    {
        $mail = new Mail($customerName->fullName(), $email);

        // send mail
    }