查找前一个兄弟节点，当前节点位置为0和1的时候都行，但是2不行，求帮忙
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>nextSibling</title>
</head>
<body>
<ul id="u1">
<li id="a">javascript</li>
<li id="b">jquery</li>
<li id="c">html</li>
</ul>
<ul id="u2">
<li id="d">css3</li>
<li id="e">php</li>
<li id="f">java</li>
</ul>
<script type="text/javascript">
function get_nextSibling(n){
var x=n.nextSibling;
while (x && x.nodeType!=1){
x=x.nextSibling;
}
return x;
}
function get_previousSibling(n){
var x=document.getElementsByTagName("li")[0];
var y=x.nextSibling;
while(x!=n&&x.nextSibling!=n&&y.nextSibling!=n){
document.write("<br>"+x.innerHTML+" "+y.innerHTML);
if(y.nodeType!=1){
document.write("1");
x=x.nextSibling;
}
else{
y=x.nextSibling;
}
}
return x;
}
var x=document.getElementsByTagName("li")[2];
document.write(x.nodeName);
document.write(" = ");
document.write(x.innerHTML);
//var y=get_nextSibling(x);
var y=get_previousSibling(x);
if(y!=x){
document.write("<br />nextsibling: ");
document.write(y.nodeName);
document.write(" = ");
document.write(y.innerHTML);
}else{
document.write("<br>已经第一个节点");
}
</script>
</body>
</html>
