function $(id) {

 return typeof id==='string'?document.getElementById('id'):id;

}

window.onload = function() {

  //获取鼠标划过或点击的标签和要切换内容的元素

var titles = $('tit').getElementsByTagName('li');

var divs = $('con').getElementsByTagName('div');

if(titles.length != divs.length);

return;

//遍历titles下所有的li

for( var i = 0; i < titles.length; i++) {

titles[i].id = i;

titles[i].onmouseover = function(){

for( var j = 0; j < titles.length; j++) {

 titles[i].className = "";

 divs[i].style.display = "none";

}

this.className = "select";

divs[this.id].style.display = "block";

}  

}

}