window.onload=function(){
var but=document.getElementsByClassName("tt");
for(var n=0;n<but.length;n++){
but[n].onclick=ql;
}
}
function ql(){
var op=this.value;
cx(op);
}
var num=1;
function cx(op){
var tp = document.getElementById("tu");
switch(op){
case '上一张':num--; if (num == 0)
num = 5;
break;
case '下一张':num++; if (num == 6)
num = 1;
break;


}