生成随机字符串
function randomStr(len) {
var str = "";
var possible = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
for( var i=0; i < len; i++ )
str += possible.charAt(Math.floor(Math.random() * possible.length));
return str;
}
