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                        /前端開發
                        
                            /JavaScript
                        
                        /瀑布流布局

為什么我的是0？請大神指出錯誤

<!doctype html>
<html>
<head>
??? <meta charset="UTF-8">
?? ?<title>Document</title>
</head>
<body>
?? ??? ?<ul id="list">
?? ??? ??? ?<li>1</li>
?? ??? ??? ?<li>2</li>
?? ??? ??? ?<li>3</li>
?? ??? ??? ?<li>4</li>
?? ??? ?</ul>
?? ??? ?<p>class為on的p標簽</p>
</body>
</html>
<script type="text/javascript">
? window.onload=function(){
???? var len1= getByClass('on','list');
???? alert(len1.length);? // 結果等于2為正確
???? var len2= getByClass('on');
???? alert(len2.length);? // 結果等于3為正確
}
function getByClass(clsName, parent){
?//定義函數getByClass()實現獲取document或指定父元素下所有class為on的元素 ?
??? var oP=parent?document.getElementById(parent):document,
??????? box=new Array();
??????? s = oP.getElementsByTagName('*');
??? for(i=0;i<s.length;i++){
??????? if(s[i].className==clsName){
??????????? box.push(s[i]);
??????? }
??? }
??? return box;
}
</script>