基礎類方法：生成隨機驗證碼
codeLength驗證碼長度
+ (NSString *)randomCode:(int)codeLength
{
CFUUIDRef uuidObj = CFUUIDCreate(nil);
NSString *radomString = [(NSString*)CFBridgingRelease(CFUUIDCreateString(nil, uuidObj)) substringToIndex:codeLength];
CFRelease(uuidObj);
return radomString;
}
