def read_file(number):
    """ Read puzzle by its number
    :param number: the number of the puzzle
    :return: the board
    """
    missing_zeros = 3 - len(str(number))
    filename = "puzzles/" + ("0" * missing_zeros) + str(number) + ".txt"
    file = open(filename, 'r')

    lines = file.readlines()

    board = []

    for y in range(len(lines)):
        board.append(lines[y][0:-1])

    return board