// 生成 "0000-9999" 4位驗證碼    int num = (arc4random() % 10000);    randomNumber = [NSString stringWithFormat:@"%.4d", num];    NSLog(@"%@", randomNumber);// 生成 "000000-999999" 6位驗證碼    int num = (arc4random() % 1000000);    randomNumber = [NSString stringWithFormat:@"%.6d", num];    NSLog(@"%@", randomNumber);