def flatten_list(lists):
    """
    Returns a flatten list from a list of lists.
        INPUT: [[1,2,3,4], [1,2,3], [3]]
        OUTPUT: [1, 2, 3, 4, 1, 2, 3, 3]
    :param lists: a 2D list
    :return: a flatten list with all those elements in one list.
    """
    flat_list = []
    for sublist in lists:
        for item in sublist:
            flat_list.append(item)

    return flat_list