/**
 * 递归法实现的快速排序
 * @param $seq
 * @return array
 */
function quicksort($seq)
{
    if (count($seq) > 1) {
        $k = $seq[0];
        $x = array();
        $y = array();
        $_size = count($seq); //do not use count($seq) in loop for.
        for ($i = 1; $i < $_size; $i++) {
            if ($seq[$i]