这个题怎么做？详细点 ，谢谢大家

[cstpwxy]

秒数显示：文本框
按钮（开始计时）按钮（停止计时）

[monexus]

<script type="text/javascript">
	var tid;
	function timer(){
		var ret = document.getElementById('ret');
		if (tid) {
			clearInterval(tid);
			tid = null;
		} else {
			tid = setInterval(function(){
				ret.value = parseInt(ret.value) + 1;
			}, 100);
		}
	}
</script>
<input id="ret" type="text" value="0" readonly="true" />
<input type="button" value="Start" onclick="timer()"/>
<input type="button" value="Stop" onclick="timer()"/>

[hookee]

<script>
var tid;
function show(){
  var n = parseInt(document.forms[0].pad.value);
  document.forms[0].pad.value = n+1;	
}
function doStart(){
  document.forms[0].pad.value="0";
  if(tid!=null){clearInterval(tid);tid=null;}
  tid=setInterval("show()",1000);
}
function doEnd(){
  
  if(tid!=null){clearInterval(tid);tid=null;}
}

</script>

<form>
秒数显示	<input type="text" name="pad" value="0"><br>
<input type="button" onclick="doStart();" value="开始计时">
<input type="button" onclick="doEnd();" value="结束计时">

</form>

[浪尖赏花]

<input id=s type=text value="0">
<input type="button" id="start" onclick="startCount()" value="start" disabled/>
<input type="button" id="stop" onclick="stopCount()" value="stop"/>
<script>
var flag = true;
window.onload = function (){
window.setInterval("count()",1000);
};
function startCount(){
flag = true;
document.getElementById("start").disabled = true;
document.getElementById("stop").disabled = false;
}
function stopCount(){
flag = false;
document.getElementById("start").disabled = false;
document.getElementById("stop").disabled = true;
}
function count(){
if(flag){
document.getElementById("s").value = parseInt(document.getElementById("s").value)+1;
}
}
</script>

[chen_ya_ping]

var s;
var seconds=0;
function StartTime() {
s=window.setInterval("StartCount()",1000);
}
function StartCount() {
seconds++;
document.getElementById("Text1").value = seconds;
}
function StopTime()
{
window.clearInterval(s);
}

<input id="Text1" type="text" value=0 />秒
<br />

<input id="Button1" type="button" value="Start" onclick="StartTime()" />  
   
<input id="Button2" type="button" value="Stop" onclick="StopTime()" />