<!DOCTYPE html>
<html>
<head>
</head>
<body>
<p id="calc"></p>
<script>
function rev_num(n)
{
return n < 10 ? n : parseInt(n % 10) * Math.pow(10, _log10(n)) + rev_num(parseInt(n / 10));
}
function _log10(n)
{
return parseInt( Math.floor( (Math.log(n) / Math.LN10) ));
//return parseInt( Math.floor( (Math.log(n) / Math.log(10)) ));
}
function print_rev_num(n)
{
document.getElementById("calc").innerHTML = rev_num(n);
}
</script><input type="button" value="rev_num(123)" onclick="print_rev_num(123)" />
<!--
(n % 10) * pow(10, (int)log10(n)) + rev_num(n / 10);
3 * 10 ^ 2 = 300 + rev_num(12)
2 * 10 ^ 1 = 20 + rev_num(1)
1 * 10 ^ 0 = 1 + rev_num(0)
300 + 20 + 1 = 321;
-->
<input type="button" value="rev_num(1234)" onclick="print_rev_num(1234)" />
<!--
(n % 10) * pow(10, (int)log10(n)) + rev_num(n / 10);
4 * 10 ^ 3 = 4000 + rev_num(123)
3 * 10 ^ 2 = 300 + rev_num(12)
2 * 10 ^ 1 = 20 + rev_num(1)
1 * 10 ^ 0 = 1 + rev_num(0)
4000 + 300 + 20 + 1 = 4321;
-->
<input type="button" value="rev_num(1221)" onclick="print_rev_num(1221)" />
</body>
</html>
<!--
run:
321
4321
1221
-->
