原理：主要运用z-index这个属性来设置图片的展示和隐藏，代码如下：
1 <!DOCTYPE html>
2 <html lang="en">
3 <head>
4 <meta charset="UTF-8">
5 <title>zIndex制作轮播图</title>
6 <style>
7 html,body,ul,li,input{margin: 0;padding: 0}
8 .box1{
9 width: 520px;
10 height: 280px;
11 margin: 0 auto;
12 position: relative;
13 border:1px solid black;
14 }
15 ul{
16 width: 520px;
17 height: 280px;
18 position: relative;
19 }
20 ul li{
21 position: absolute;
22 list-style: none;
23 }
24 ul li a{
25 display: block;
26 width: 520px;
27 height: 280px;
28 }
29 .btn1{
30 position: absolute;
31 width: 48px;
32 height: 60px;
33 background: url(btn.png) no-repeat;
34 border:0;
35 left: 0;
36 top:110px;
37 cursor: pointer;
38 z-index: 10000;
39 }
40 .btn2{
41 position: absolute;
42 width: 48px;
43 height: 60px;
44 background: url(btn.png) no-repeat -48px;
45 border:0;
46 right: 0;
47 top:110px;
48 cursor: pointer;
49 z-index: 10000;
50 }
51 </style>
52 <script>
53 window.onload = function(){
54 var btn1 = document.getElementsByClassName("btn1")[0];
55 var btn2 = document.getElementsByClassName("btn2")[0];//有兼容问题
56 var liList = document.getElementsByTagName("ul")[0].children;
57 var index = 1;
58 var i = 4;
59 btn1.onclick = function(){
60 i++;
61 if(i == 5){
62 i = 0;
63 }
64 liList[i].style.zIndex = index++;
65 }
66 btn2.onclick = function(){
67 i--;
68 if(i == -1){
69 i = 4;
70 }
71 liList[i].style.zIndex = index++;
72 }
73 var timer = setInterval(function(){btn2.onclick();},1000)
74 }
75 </script>
76 </head>
77 <body>
78 <div class="box1">
79 <ul>
80 <li><a href=""><img src="5.jpg" alt=""></a></li>
81 <li><a href=""><img src="4.jpg" alt=""></a></li>
82 <li><a href=""><img src="3.jpg" alt=""></a></li>
83 <li><a href=""><img src="2.jpg" alt=""></a></li>
84 <li><a href=""><img src="1.jpg" alt=""></a></li>
85 </ul>
86 <input type="button" name="" class="btn1" />
87 <input type="button" name="" class="btn2" />
88 </div>
89 </body>
90 </html>
问题：在这里我们加了一个定时器来让图片自己切换，当我们点击按钮的时候需要关闭定时器，这个可以通过clearInterval(timer)来实现，但是当我们不点击按钮的时候，想让定时器接着我们现在的图片自动轮播下去，这个问题如何解决呢？重新开定时器吗？