一道算法题，以前51js上讨论过。思路就是自己实现乘法运算，如果用递归，如果求1000的阶乘就要出现脚本失控了
51JS上最为经典的方法：
1: <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
2: <html xmlns="http://www.w3.org/1999/xhtml">
3: <head>
4: <title> new document </title>
5: <meta name="generator" content="editplus" />
6: <meta name="author" content="" />
7: <meta name="keywords" content="" />
8: <meta name="description" content="" />
9: </head>
10:
11: <body>
12: <script type='text/javascript'>
13: function factorial(n) {
14: var a = [1];
15: for (var i = 1;i<=n ;i++) {
16: for (var j = 0, c = 0;j<a.length || c != 0;j++ ) {
17: var m = (j < a.length) ? (i*a[j] + c) : c;
18: a[j] = m % 10;
19: c = (m - a[j]) / 10;
20: }
21: }
22:
23: return a.reverse().join("");
24: }
25:
26: var num = 1000;
27:
28: alert(factorial(num));
29: </script>
30: </body>
31: </html>
