function qsort(&$arr)  {      _quick_sort($arr, 0, count($arr) - 1);  }    /**   * 采用递归算法的快速排序。   *   * @param array $arr 要排序的数组   * @param int $low  最低的排序子段   * @param int $high 最高的排序字段   */  function _quick_sort(&$arr, $low, $high)  {       $low_data = $arr[$low];       $prev_low = $low;       $prev_high = $high;       while ($low < $high)        {              while ($arr[$high] >= $low_data && $low < $high) {                  $high--;              }              if ($low < $high) {                  $arr[$low] = $arr[$high];                  $low++;              }              while ($arr[$low] <= $low_data && $low < $high) {                  $low++;              }              if ($low < $high) {                  $arr[$high] = $arr[$low];                  $high--;              }       }       $arr[$low] = $low_data;       if ($prev_low < $low) {           _quick_sort($arr, $prev_low, $low);       }       if ($low + 1 < $prev_high) {           _quick_sort($arr, $low + 1, $prev_high);       }  }    function quick_sort(&$arr)  {      $stack = array();      array_push($stack, 0);      array_push($stack, count($arr) -1);      while (!empty($stack)) {          $high = array_pop($stack);          $low = array_pop($stack);          $low_data = $arr[$low];          $prev_low = $low;          $prev_high = $high;          while ($low < $high)           {              while ($arr[$high] >= $low_data && $low < $high) {                  $high--;              }              if ($low < $high) {                  $arr[$low] = $arr[$high];                  $low++;              }              while ($arr[$low] <= $low_data && $low < $high) {                  $low++;              }              if ($low < $high) {                  $arr[$high] = $arr[$low];                  $high--;              }          }          $arr[$low] = $low_data;          if ($prev_low < $low) {              array_push($stack, $prev_low);              array_push($stack, $low);          }          if ($low + 1 < $prev_high) {              array_push($stack, $low + 1);              array_push($stack, $prev_high);          }      }  }