Проверить правильность расстановки костяшек домино

public class Domino {
    private static final char EMPTY = '\u00D8';
 
    private char left;
    private char right;
 
    public static Domino fromInt(final int value) {
        if ((value > 66) || (value < 0))
            throw new IllegalArgumentException("Value " + value + " is not in range [0..66]");
 
        return new Domino(encode(value / 10), encode(value % 10));
    }
 
    public boolean isAttachableTo(final Domino next) {
        return this.right == next.left;
    }
 
    @Override
    public String toString() {
        return "[" + left + "-" + right + "]";
    }
 
    private Domino(char left, char right) {
        this.left = left;
        this.right = right;
    }
 
    private static char encode(final int value) {
        if (value == 0)
            return EMPTY;
        else
            return (char) (value + 48);
    }
}

public class Main {
    public static void main(String[] args) {
        Integer a[] = { 13, 34, 44, 46, 66, 63, 33, 32, 21, 10, 0, 1, 14 };
        Integer b[] = { 43, 31, 10, 0, 4, 44, 45, 51, 22 };
        Integer c[] = { 13, 34, 44, 46, 66, 63, 66, 32, 21, 10, 0, 1, 14 };
        Integer d[] = { 13, 34, 44, 46, 66, 63, 32, 21, 10, 0, 1, 14, -2 };
 
        testSequence(a);
        testSequence(b);
        testSequence(c);
        testSequence(d);
    }
 
    private static void testSequence(final Integer ... a) {
        Optional<String> failure = validateSequence(a);
 
        if (failure.isPresent())
            System.out.println("Sequence is invalid: " + failure.get());
        else
            System.out.println("Sequence is valid");
    }
 
    private static Optional<String> validateSequence(final Integer ... sequence) {
        String failureReason = null;
 
        final List<Integer> items = Arrays.asList(sequence);
 
        final Set<Integer> duplicates = items.stream()
                .filter(x -> Collections.frequency(items, x) > 1)
                .collect(Collectors.toSet());
 
        if (duplicates.size() > 0) {
            final StringJoiner stringJoiner = new StringJoiner(", ", "Items: ", " appear more than once");
 
            duplicates.forEach(item -> stringJoiner.add(Domino.fromInt(item).toString()));
 
            failureReason = stringJoiner.toString();
        }
 
        int i = 0;
        while ((i < items.size() - 1) && (failureReason == null)) {
            try {
                final Domino left = Domino.fromInt(items.get(i));
                final Domino right = Domino.fromInt(items.get(i + 1));
 
                if (!left.isAttachableTo(right))
                    failureReason = left + " cannot be attached to " + right;
 
            } catch (IllegalArgumentException e) {
                failureReason = e.getMessage();
            }
 
            i++;
        }
 
        return Optional.ofNullable(failureReason);
    }
}