Javascript 获取LI里的内容

代码如下:
<strong>数据源:</strong>
<ul>
<li>脚本之家</li>
<ul>
<li>a_1</li>
<li>a_2</li>
<li>a_3</li>
</ul>
<li>www.gxlcms.com</li>
<li>www.kanshule.com</li>
</ul>

<script>
document.write("<strong>取外层ul的li信息:</strong><br>");
var tree1 = document.getElementsByTagName("ul")[0].childNodes;
for (var i = 0;i < tree1.length;i++){
document.write(tree1[i].innerHTML);
}

document.write("<br><br><strong>取内层ul的li信息:</strong><br>");

var tree2 = document.getElementsByTagName("ul")[1].childNodes;
for (var i = 0;i < tree2.length;i++){
document.write(tree2[i].innerHTML);
}

</script>