- (NSString *)normalizeVietnameseString:(NSString *)str {
    NSMutableString *originStr = [NSMutableString stringWithString:str];
    CFStringNormalize((CFMutableStringRef)originStr, kCFStringNormalizationFormD);

    CFStringFold((CFMutableStringRef)originStr, kCFCompareDiacriticInsensitive, NULL);

    NSString *finalString1 = [originStr stringByReplacingOccurrencesOfString:@"đ"withString:@"d"];

    NSString *finalString2 = [finalString1 stringByReplacingOccurrencesOfString:@"Đ"withString:@"D"];
    return finalString2;
}