$category = DB::name('Category');
$category_list = $category->select();
$data=$this->list_to_tree($category_list,'category_id','category_parent_id','children'); //调用下面的方法
$data=$this->findChild($data);
header('Content-Type:application/json; charset=utf-8');
exit(json_encode($data,JSON_UNESCAPED_UNICODE));
// return $this->fetch('index');
$this->ajaxReturn($data, 'JSON');

  function findChild($arr){
    static $tree=array();
    foreach ($arr as $key=>$val){
        $tree[]=$val;
        if (isset($val['_child'])){
          $this->findChild($val['_child']);
        }
      }
  return $tree;
  }
  /**
   * 把返回的数据集转换成Tree
   * @access public
   * @param array $list 要转换的数据集
   * @param string $pid parent标记字段
   * @param string $level level标记字段
   * @return array
   */
  function list_to_tree($list, $pk='id',$pid = 'pid',$child = '_child',$root=0) {
    // 创建Tree
    $tree = array();
    if(is_array($list)) {
    // 创建基于主键的数组引用
    $refer = array();
    foreach ($list as $key => $data) {
      $refer[$data[$pk]] =& $list[$key];
    }
    foreach ($list as $key => $data) {
      // 判断是否存在parent
      $parentId = $data[$pid];
      if ($root == $parentId) {
      $tree[] =& $list[$key];
      }else{
      if (isset($refer[$parentId])) {
        $parent =& $refer[$parentId];
        $parent[$child][] =& $list[$key];
      }
      }
    }
    }
    return $tree;
  }