简单的javascript实现下拉联动效果

<div class="cont_left" onload="javascript:sethdHelper('productServer','PDSL',1,6)">
<ul class="menu">
<li class="level1" id="productServer_1"><a id="PDSL_1" href="javascript:sethdHelper('productServer','PDSL',1,6)">
产品说明</a>
<ul class="level2 tab" id="lists_productServer_1" style="display: none;">
<li><a href="HelpContent.aspx?lk=1&hId=119">产品特色</a></li>
<li><a href="HelpContent.aspx?lk=1&hId=120">产品构成</a></li>
<li><a href="HelpContent.aspx?lk=1&hId=121">服务模式</a></li>
</ul>
</li>
<li class="level1" id="productServer_2"><a id="PDSL_2" href="javascript:sethdHelper('productServer','PDSL',2,6)">
产品购买指南</a>
<ul class="level2 tab" id="lists_productServer_2" style="display: none;">
<li><a href="HelpContent.aspx?lk=2&hId=122">如何购买</a></li>
<li><a href="Pro_Order.aspx?id=115&lk=2">在线订购</a></li>
<li><a href="Pro_Pay.aspx?id=115&lk=2">付款方式</a></li>
<li><a href="HelpContent.aspx?lk=2&hId=123">产品列表</a></li>
<li><a href="HelpContent.aspx?lk=2&hId=124">专业软件定制</a></li>
</ul>
</li>
<li class="level1" id="productServer_3"><a id="PDSL_3" href="javascript:sethdHelper('productServer','PDSL',3,6)">
我要试用</a>
<ul class="level2 tab" id="lists_productServer_3" style="display: none;">
<li class="level1"><a href="Pro_Try.aspx?lk=3">我要试用</a> </li>
</ul>
</li>
<li class="level1" id="productServer_4"><a id="PDSL_4" href="javascript:sethdHelper('productServer','PDSL',4,6)">
代理专区</a>
<ul class="level2 tab" id="lists_productServer_4" style="display: none;">
<li><a href="HelpContent.aspx?lk=4&hId=125">代理流程</a></li>
</ul>
</li>
<li class="level1" id="productServer_5"><a id="PDSL_5" href="javascript:sethdHelper('productServer','PDSL',5,6)">
客服中心</a>
<ul class="level2 tab" id="lists_productServer_5" style="display: none;">
<li><a href="HelpContent.aspx?lk=5&hId=126">常见问题解答</a></li>
</ul>
</li>
<li class="level1" id="productServer_6"><a id="PDSL_6" href="javascript:sethdHelper('productServer','PDSL',6,6)">
联系我们</a>
<ul class="level2 tab" id="lists_productServer_6" style="display: none;">
<li><a href="HelpContent.aspx?lk=6&hId=127">联系我们</a></li>
</ul>
</li>
</ul>
</div>