def get_playlist_track_list(urm):
    """
        Returns a dictionary with:
            key: playlist id
            value: array of tracks in such playlist
        The URM is the matrix with pairs "playlist_id"x"track_id" (i.e. train_final.csv)
    """
    playlist_content = {}
    for _,row in urm.iterrows():
        playlist_id = row['playlist_id']
        if playlist_id in playlist_content:
            playlist_content[playlist_id].append(row['track_id'])
        else:
            playlist_content[playlist_id] = [row['track_id']]

    return playlist_content