css多个div浮动float高度自适应的两种方法

<style type="text/css">
<!--
#wrap{overflow:hidden;}
#sidebar_left,#sidebar_right{padding-bottom:100000px;margin-bottom:-100000px;}
-->
</style>
<div id="wrap" style="width:300px; background:#FFFF00;">
    <div id="sidebar_left" style="float:left;width:100px; height:1000px; background:#FF0000;">Left</div>
    <div id="sidebar_mid" style="float:left;width:100px; background:#666;">
    Middle<br />
    Middle<br />
    Middle<br />
    Middle<br />
    Middle<br />
    Middle<br />
    Middle<br />
    Middle<br />
    Middle<br />
    </div>
    <div id="sidebar_right" style="float:right;width:100px; height:500px; background:#0000FF;">Right</div>
</div>

<script>
var a=Math.max(document.getElementById("left").offsetHeight,document.getElementById("center").offsetHeight,document.getElementById("right").offsetHeight);  //获取3个div的最大高度
document.getElementById("left").style.height = a + "px";
document.getElementById("center").style.height = a + "px";
document.getElementById("right").style.height = a + "px";
</script>