怎么用js精确判断li已经在ul存在过了?

<ul class="memory_messagelist" id="memory_messagelist">

<li><span>我是1</span><span>1留言</span></li>

<li><span>我是2</span><span>2留言</span></li>

<li><span>我是3</span><span>3留言</span></li>

<li><span>我是4</span><span>4留言</span></li>

<li><span>我是5</span><span>5留言</span></li>

</ul>

<script>

var data="3留言";

var ymylis=document.getElementById('memory_messagelist').innerHTML;

if(ymylis.toString().indexOf(data)>-1)

alert("有了");

else

alert("木有");

</script>