一各非常棘手的问题，高手请进！！！！

<HTML>
<HEAD>
<META NAME="GENERATOR" Content="Microsoft Visual Studio 6.0">
<TITLE></TITLE>
</HEAD>
<BODY><TABLE id="a1" WIDTH=75% BORDER=1 CELLSPACING=1 CELLPADDING=1>
<TR>
<TD colspan="2">1</TD>
<TD>3</TD>
</TR>
<TR>
<TD>1</TD>
<TD colspan="2">2</TD>
</TR>
<TR>
<TD>1</TD>
<TD>2</TD>
<TD>3</TD>
</TR>
</TABLE>
</BODY>
</HTML>
<SCRIPT LANGUAGE=javascript>
<!--
var colLength = 0; //获得table的列数，注意有可能有些列跨列
for (var i=0;i< a1.rows(0).cells.length;i++){
colLength += a1.rows(0).cells(i).colSpan;
}a1.insertRow(0);
a1.rows(0).insertCell();
a1.rows(0).cells(0).colSpan = colLength;
a1.rows(0).cells(0).innerHTML = "表头";a1.insertRow();
a1.rows(a1.rows.length-1).insertCell();
a1.rows(a1.rows.length-1).cells(0).colSpan = colLength;
a1.rows(a1.rows.length-1).cells(0).innerHTML = "表尾";
//-->
</SCRIPT>