同行内两个<li>隔行变色

<style>
ul{
 60px
}
li{
 margin:0px;
 padding:0px;
 list-style:none;
 display:inline;
 30px
}
</style>
<ul id="test">
<li>1</li>
<li>2</li>
<li>3</li>
<li>4</li>
<li>5</li>
<li>6</li>
<li>7</li>
<li>8</li>
<li>9</li>
<li>10</li>
<li>11</li>
<li>12</li>
</ul>
<script>
var obj=document.getElementById("test").getElementsByTagName("li"),j=obj.length;
for (var i=0;i<j;i+=2){
if (cu(i)=="r")obj[i].style.background=obj[i+1].style.background="#f00"
if (cu(i)=="y")obj[i].style.background=obj[i+1].style.background="#ff0"
}
function cu(k){
if (k%2==0 && (k/2)%2==0)return "r"
return "y"
}
</script>