How can I load jQuery if it is not already loaded?

Sometimes need to check that in a page jquery already loaded then no need extra jquery or not loaded then need to load jquery, you can do it using simple jquery code. Here is the code

<script type="text/javascript">
 var baseurl="shafiqul.wordpress.com";
 var doc=document.getElementsByTagName("head").item(0);
 var jQueryScriptOutputted = false;
 var jQueryScriptOutputtedplaceholder = false;
 window.onload=function()
 {
 initJQuery();
 }
 function initJQuery() {
 if (typeof(jQuery) == "undefined") {
 if (! jQueryScriptOutputted) {
 jQueryScriptOutputted = true;
 includeJs("/js/jquery-1.4.4.min.js", true);
| }
 }
 if (! jQueryScriptOutputtedplaceholder) {
 jQueryScriptOutputtedplaceholder = true;
 includeJs("/scripts/html5_placeholder.js", true);
 }
 setTimeout("initJQuery()", 100);
 }
 function includeJs(fileName, jQ, head){
 var js=document.createElement("script");
 js.setAttribute("language", "javascript");
 js.setAttribute("type", "text/javascript");
 js.setAttribute("src", baseurl+fileName);
 if(jQ===true){
 doc.insertBefore(js, doc.firstChild);
 return false;
 }
 if(head===true){
 doc.appendChild(js);
 return false;
 }
 document.getElementsByTagName("body").item(0).appendChild(js);
 return false;
 }
 </script>

if you find any problem then please give a comment.