PHP兑现快速排序算法

<?php
function quicksort($seq) {
? if (count($seq) > 1) {
??? $k = $seq[0];
??? $x = array();
??? $y = array();
??? for ($i=1; $i<count($seq); $i++) {
????? if ($seq[$i] <= $k) {
??????? $x[] = $seq[$i];
????? } else {
??????? $y[] = $seq[$i];
????? }
??? }
??? $x = quicksort($x);
??? $y = quicksort($y);
??? return array_merge($x, array($k), $y);
? } else {
??? return $seq;
? }
}

$arr = array(12,2,16,30,8,28,4,10,20,6,18);
print_r(quicksort($arr));
?>