解决ie document.getElementsByClassName失效方法

if(!document.getElementsByClassName){
        document.getElementsByClassName=function(cls){
            var ret=[];
            var els=document.getElementsByTagName("*");
            for(var i=0;i<els.length;i++){
                if(els[i].className===cls ||els[i].className.indexOf(cls)>=0 ||els[i].className.indexOf(cls+" ") ||els[i].className.indexOf(" "+cls+" ") ||els[i].className.indexOf(" "+cls)){
                    ret.push(els[i]);
                }
            }
            return ret;
        }
    }