无限极递归优化

分享一个不错的递归方式，效率更高

public function departmentTree($data)
 {
     $items = array();
     foreach ($data as $v) {
         $items[$v['id']] = $v;
     }
     $tree = array();
     foreach ($items as $k => $item) {
         if (isset($items[$item['parent_id']])) {
             $items[$item['parent_id']]['level'][] = &$items[$k];
         } else {
             $tree[] = &$items[$k];
         }
     }
     return $tree;
 }