如何生成所有可能的有效括号组合？

<?php
function generateParenthesis($n) {
    $result = [];
    $stack = [];
    $leftCount = 0;
    $rightCount = 0;

    function backtrack($leftCount, $rightCount, $stack, &$result) {
        if ($leftCount === $n && $rightCount === $n) {
            $result[] = implode('', $stack);
            return;
        }

        if ($leftCount < $n) {
            $stack[] = '(';
            backtrack($leftCount + 1, $rightCount, $stack, $result);
            array_pop($stack); // 回溯：移除最后添加的左括号
        }

        if ($rightCount < $leftCount) {
            $stack[] = ')';
            backtrack($leftCount, $rightCount + 1, $stack, $result);
            array_pop($stack); // 回溯：移除最后添加的右括号
        }
    }

    backtrack($leftCount, $rightCount, $stack, $result);
    return $result;
}

$n = 3;
$combinations = generateParenthesis($n);
foreach ($combinations as $combination) {
    echo $combination . PHP_EOL;
}
?>

((()))
(()())
(())()
()(())
()()()