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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_hour_dirs(root=None):
 """Gets the directories under selfplay_dir that match YYYY-MM-DD-HH."""

    root = root or selfplay_dir()
    return list(filter(lambda s: re.match(r"\d{4}-\d{2}-\d{2}-\d{2}", s),
                       gfile.ListDirectory(root)))