请老师检查我的代码，轮播效果没有

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>综合案例</title>
<link rel="stylesheet" type="text/css" href="综合案例.css">
<body>
<div class="main">
  <div class="banner">
   <a href="javascript:void();">
    <div class="banner-slide slide1 slide-active"></div>
   </a>
   <a href="">
    <div class="banner-slide slide2 "></div>
   </a>
   <a href="">
    <div class="banner-slide slide3"></div>
   </a>
  </div>

   <a class=" button prev"></a>>
   <a class=" button next"></a>

  <div class="dots">
   <span class="active"></span>
   <span></span>
   <span></span>
  </div>
</div>
<script src="综合案例.js"> </script>
</body>
</html>

*{
margin: 0;
padding: 0;
}
body{
font-family: "微软雅黑";
color: #14191e;
}
.main{
width: 1200px;
height: 460px;
overflow: hidden;
margin: 30px auto;
position: relative;
}
.banner{
width: 1200px;
height: 460px;
overflow: hidden;
position: relative;
}
.banner-slide{
width: 1200px;
height: 460px;
background-repeat: no-repeat;
position: absolute;
display: none;
}
.slide1{
background-image:url(img\\bg1.jpg);
}

.slide2{
background-image:url(img\\bg2.jpg);
}

.slide3{
background-image:url(img\\bg3.jpg);
}
.slide-active{
display: block;
}
.button{
position: absolute;
width: 40px;
height: 80px;
left: 244px;
top: 50%;
margin-top: -40px;
background: url("img\\arrow.png") no-repeat center center;
/*background-color: gray;*/

}
.prev{
transform: rotate(180deg);
}
.next{
right: 0;
left: auto;
}
.button:hover{
background-color: #333;
opacity: 0.5;
}
.dots{

position: absolute;
bottom: 20px;
right: 0;
text-align: right;
margin-right: 30px;
}
.dots span{
   width: 12px;
   height: 12px;
   border-radius: 50%;
   line-height: 12px;
   /*background-color:black;
   opacity: 0.4;*/
   background:rgba(7,17,27,0.4);
/*添加阴影*/
   box-shadow: 0 0 0 1px rgba(255,255,255,0.8) inset;
   display: inline-block;
margin-left: 10px;
cursor: pointer;/*光标形状*/
}
.active{
background: #fff;
box-shadow: 0 0 0 1px rgba(7,17,27,0.4) inset;
}

//封装getElementById()
  function byId(id){
      return   typeof(id)==="String"?document.getElementById('id'):id;
  }
        var timer=null;
        var    pics=byId("banner").getElementsByTagName("div");
        var   len=pics.length;
        var    index=0;
        function slideImg(){
//取出mian元素
    var main=byId("main");

//鼠标放置在图片上
       main.onmouseover=function(){
         if(timer)
       clearInterval(timer);
       }
//鼠标从图片上移走时，调用onmouseout事件
      main.onmouseout=function(){
       timer= setInterval(function(){
        index++;
        if(index==len)
            index=0;
        //切换图片
        changeImg();
        },2000);
      }
       //打开页面自动执行图片轮播,调用onmouseout方法
       main.onmouseout();
}

//切换图片
     changeImg()
     {
        //遍历banner里所有的div,将其隐藏
    for(var i=0;i<len;i++)
        pics[i].style.display="none";
    //根据索引找到当前div,将其显示出来
    pics[index].style.display='block';
       }
      slideImg();