PHP嵌套循环输出导航[不使用递归]

<?php// 本类由系统自动生成，仅供测试用途class TestAction extends Action {    public function index(){	$select=M('select');		$rs=$select->where('parent_id=0')->select();		//$rs=$select->where('parent_id='.$parentid)->select();		for($i=0;$i<count($rs);$i++){			if($i==0){				echo '<p class="letter" id="'.$rs[$i]['index'].'">'.$rs[$i]['index'].'</p>';				$index=$rs[$i]['index'];			}			//去重输出首字母			if($rs[$i]['index']!=$index){				echo '<p class="letter" id="'.$rs[$i]['index'].'">'.$rs[$i]['index'].'</p>';				$index=$rs[$i]['index'];			}			echo $rs[$i]['name'].'<br>';			$series=$select->where('parent_id='.$rs[$i]['id'])->select();			for($j=0;$j<count($series);$j++){				echo $series[$j]['name'].'<br>';				$model=$select->where('parent_id='.$series[$i]['id'])->select();				for($x=0;$x<count($model);$x++){					echo $model[$x]['name'].'<br>';				}				//for()			}		}    }}