随机数生成（数字，字母）

/// 输入位数 生成随机数

+ (NSString *)xpf_generateArc4randomStr:(int)index {

    NSString *str = @"";

    for (int i = 0; i < index; i++) {

        int a = arc4random() % 10;

        str = [NSString stringWithFormat:@"%@%d", str, a];

    }

    return str;

}

/// 返回 几位大小写字母和数字

+ (NSString *)xpf_returnIndexLetterAndNumber:(NSUInteger)index {

    //定义一个包含数字，大小写字母的字符串

    NSString *strAll = @"0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";

    //定义一个结果

    NSString *result = [[NSMutableString alloc] initWithCapacity:index];

    for (int i = 0; i < index; i++) {

        //获取随机数

        NSInteger index = arc4random() % (strAll.length - 1);

        char tempStr = [strAll characterAtIndex:index];

        result = (NSMutableString *)[result stringByAppendingString:[NSString stringWithFormat:@"%c", tempStr]];

    }

    return result;

}