带箭头的轮播

var sy = 0;
var zd = document.getElementsByClassName('yz1').length-1;

function Huan()
{
var attr = document.getElementsByClassName('yz1');

for(var i=0;i<attr.length;i++)
{
attr[i].style.display="none";
}
attr[sy].style.display = "block";
}

function aaaa()
{
if(sy>1)
{
sy=sy-1;
}
else
{
sy = zd;
}

Huan();
}

function bbbb()
{
if(sy<zd)
{
sy=sy+1;
}
else
{
sy=0;
}
Huan();
}