如何基于insertBefore制作循环插空效果？

<ul class="list" id="list">
 <li class="in">1</li>
 <li class="in">2</li>
 <li class="in">3</li>
 <li class="in">4</li>
 <li class="in">5</li>
 <li class="in">6</li> 
</ul>

.in{
 height: 20px;
 line-height: 20px;
 width: 20px;
 background-color: blue;
 text-align: center;
 color: white;
}

var oList = document.getElementById('list');
//新增一个li元素
var oAdd = document.createElement('li');
//设置新增元素的css样式
oAdd.className = "in";
oAdd.style.cssText = 'background-color:red;border-radius:50%';
//添加到oList中
oList.insertBefore(oAdd,null);
var num = -1;
var max = oList.children.length;
function incrementNumber(){
 num++;
 //oList.getElementsByTagName('li')[max]相当于null，所以不报错
 oList.insertBefore(oAdd,oList.getElementsByTagName('li')[num]); 
 if(num == max){
 num = -1;
 } 
 if(num == 0){
 num = 1;
 }
 setTimeout(incrementNumber,1000);
}
setTimeout(incrementNumber,1000);