根据ID动态改变菜单点击后样式

<script type="text/javascript">
      function getgroupBg(num)
    {
        for(var id = 0;id<=6;id++)
        {
            if(id==num)
            {
                document.getElementById("mynav"+id).className="nav_a";
            }
            else
            {
                document.getElementById("mynav"+id).className="";
            }
        }
    }
</script>
<!--页面底部代码：-->
<script type="text/javascript">
document.getElementById("mynav0").className="nav_a";
</script>
<!--导航代码：-->
<ul style="margin-top:0px;">
<li><a class="haha" id="mynav0" onClick="getgroupBg(0);" href="javaScript:void(0)">000</a></li>
<li><a class="haha" id="mynav1" onClick="getgroupBg(1);" href="javaScript:void(0)">111</a></li>
<li><a class="haha" id="mynav2" onClick="getgroupBg(2);" href="javaScript:void(0)">222</a></li>
<li><a class="haha" id="mynav3" onClick="getgroupBg(3);" href="javaScript:void(0)">333</a></li>
<li><a class="haha" id="mynav4" onClick="getgroupBg(4);" href="javaScript:void(0)">444</a></li>
<li><a class="haha" id="mynav5" onClick="getgroupBg(5);" href="javaScript:void(0)">555</a></li>
<li><a class="haha" id="mynav6" onClick="getgroupBg(6);" href="javaScript:void(0)">666</a></li>
</ul>