这里有3个问题：aLi[i].onmouseover=function(){ if(sub){
                var that =sub;
1：if(sub)这句不写行不行？
2：var that =sub;不把sub赋值给它可以吗？直接把sub写在定时器里面如sub.style.height
3:这里获取为啥不用classname啊？如：var sub=this.getElementsByClassName("subNav);
4:aLi[i].onmouseover=function(){直接sub.style.display="block;  sub.style.height=sub.offsetheight+16+"px;  }