php递归函数的例子

function factorial($n) {
    if ($n == 0) {
        return 1;
    } else {
        return $n * factorial($n - 1);
    }
}

echo factorial(5); // 输出 120

class TreeNode {
    public $value;
    public $left;
    public $right;

    public function __construct($value) {
        $this->value = $value;
        $this->left = null;
        $this->right = null;
    }
}

function preorderTraversal($root) {
    if ($root === null) {
        return [];
    }
    $result = [$root->value];
    $result = array_merge($result, preorderTraversal($root->left));
    $result = array_merge($result, preorderTraversal($root->right));
    return $result;
}

// 构建一个简单的二叉树
$root = new TreeNode(1);
$root->left = new TreeNode(2);
$root->right = new TreeNode(3);
$root->left->left = new TreeNode(4);
$root->left->right = new TreeNode(5);

print_r(preorderTraversal($root)); // 输出 [1, 2, 4, 5, 3]