李晓健
先获取所有标签，然后循环，看哪一个标签的className属性中包含有你要先的那个class;var getElementsByClassName = function (searchClass, node,tag) {
if(document.getElementsByClassName){
var nodes = (node || document).getElementsByClassName(searchClass),result = [];
for(var i=0 ;node = nodes[i++];){
if(tag !== "*" && node.tagName === tag.toUpperCase()){
result.push(node)
}else{
result.push(node)
}
}
return result
}else{
node = node || document;
tag = tag || "*";
var classes = searchClass.split(" "),
elements = (tag === "*" && node.all)? node.all : node.getElementsByTagName(tag),
patterns = [],
current,
match;
var i = classes.length;
while(--i >= 0){
patterns.push(new RegExp("(^|\s)" + classes[i] + "(\s|$)"));
}
var j = elements.length;
while(--j >= 0){
current = elements[j];
match = false;
for(var k=0, kl=patterns.length; k<kl; k++){
match = patterns[k].test(current.className);
if (!match) break;
}
if (match) result.push(current);
}
return result;
}
}