jquery左右滚动焦点图banner图片鼠标经过显示上下页按钮

<div class="bannerbox">
<div id="focus">
<ul>
<li><a href="http://www.freejs.net/" target="_blank">
<img src="../dandong.png" alt="" /></a></li>
<li><a href="http://www.freejs.net/" target="_blank">
<img src="../mohe.png" alt="" /></a></li>
<li><a href="http://www.freejs.net/" target="_blank">
<img src="../changbaishan.png" alt="" /></a></li>
<li><a href="http://www.freejs.net/" target="_blank">
<img src="../erlianhaote.png" alt="" /></a></li>
</ul>
</div>
</div>

@charset "utf-8";
img { border: 0px; }
.bannerbox { width: 320px; height: 150px; overflow: hidden; margin: 0px auto; }
#focus { width: 320px; height: 150px; clear: both; overflow: hidden; position: relative; float: left; }
#focus ul { width: 320px; height: 150px; float: left; position: absolute; clear: both; padding: 0px; margin: 0px; }
#focus ul li { float: left; width: 320px; height: 150px; overflow: hidden; position: relative; padding: 0px; margin: 0px; }
#focus .preNext { width: 250px; height: 150px; position: absolute; top: 0px; cursor: pointer; }
#focus .pre { left: 0; background: url(../images/sprite.png) no-repeat left center; }
#focus .next { rightright: 0; background: url(../images/sprite1.png) no-repeat rightright center; }

$(function () {
var sWidth = $("#focus").width();
var len = $("#focus ul li").length;
var index = 0;
var picTimer;
var btn = "<div class='btnBg'></div><div class='btn'>";
for (var i = 0; i < len; i++) {
btn += "<span></span>";
}
btn += "</div><div class='preNext pre'></div><div class='preNext next'></div>";
$("#focus").append(btn);
$("#focus .btnBg").css("opacity", 0);
$("#focus .btn span").css("opacity", 0.4).mouseenter(function () {
index = $("#focus .btn span").index(this);
showPics(index);
}).eq(0).trigger("mouseenter");
$("#focus .preNext").css("opacity", 0.0).hover(function () {
$(this).stop(true, false).animate({ "opacity": "0.5" }, 300);
}, function () {
$(this).stop(true, false).animate({ "opacity": "0" }, 300);
});
$("#focus .pre").click(function () {
index -= 1;
if (index == -1) { index = len - 1; }
showPics(index);
});
$("#focus .next").click(function () {
index += 1;
if (index == len) { index = 0; }
showPics(index);
});
$("#focus ul").css("width", sWidth * (len));
$("#focus").hover(function () {
clearInterval(picTimer);
}, function () {
picTimer = setInterval(function () {
showPics(index);
index++;
if (index == len) { index = 0; }
}, 2800);
}).trigger("mouseleave");
function showPics(index) {
var nowLeft = -index * sWidth;
$("#focus ul").stop(true, false).animate({ "left": nowLeft }, 300);
$("#focus .btn span").stop(true, false).animate({ "opacity": "0.4" }, 300).eq(index).stop(true, false).animate({ "opacity": "1" }, 300);
}
});