Javascript 获取LI里的内容

<strong>数据源:</strong> 
<ul> 
<li>三水点靠木</li> 
<ul> 
<li>a_1</li> 
<li>a_2</li> 
<li>a_3</li> 
</ul> 
<li>3water.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>