//找到最顶端的一级目录id
public function findTopPid($cid){
$f_info=M('Category')->field('pid')->find($cid);//获取当前栏目的父级id
if($f_info['pid']==0){
return $cid;
}else{
return $this->findTopPid($cid);
}
}

本来就两三次递归结果无法实现：
Maximum function nesting level of '100' reached, aborting!