def build_s3_url(filenames, bucket):
    """
    convert filenames to AWS S3 URLs
    
    params:
    bucket: string, AWS S3 bucket name
    filenames: list of strings, AWS S3 filenames
    """
    s3_urls = []
    for f in filenames:
        s3_urls.append('https://{}.s3.amazonaws.com/{}'.format(bucket, f))
    return s3_urls