One puzzle solution

Main.java

package org.toysoft;

import java.util.*;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.stream.Collectors;

public class Main {

    /**
     * Условия:
     * XX * XX - X = XXX
     * 01   23   4   567
     * Кортеж: 1 1 1 1 2 3 3 4
     */

    public static void main(String[] args) {
        List<Integer> tuple = new ArrayList<>(Arrays.asList(1, 1, 1, 1, 2, 3, 3, 4));

        List<List<Integer>> allVariants = pickAllVariants(tuple);

        AtomicInteger inc = new AtomicInteger(0);

        allVariants.stream().map(integers -> {
            inc.incrementAndGet();
            return integers;
        }).filter(Main::check).forEach(integers -> System.out.println(inc.get() + ": " + showExp(integers) + " -> right!"));

        System.out.println("Checked " + allVariants.size() + " variants.");
    }

    private static String showExp(List<Integer> tuple) {
        return String.format("%d%d * %d%d - %d = %d%d%d", tuple.toArray());
    }

    private static boolean check(List<Integer> tuple) {
        int firstNumber = tuple.get(0) * 10 + tuple.get(1);
        int secondNumber = tuple.get(2) * 10 + tuple.get(3);
        int thirdNumber = tuple.get(4);
        int resultNumber = tuple.get(5) * 100 + tuple.get(6) * 10 + tuple.get(7);

        int leftPart = firstNumber * secondNumber - thirdNumber;

        return leftPart == resultNumber;
    }

    private static List<List<Integer>> pickAllVariants(List<Integer> tuple) {
        if (tuple.size() == 1) {
            return new ArrayList<>(Collections.singletonList(tuple));
        }

        List<List<Integer>> results = new ArrayList<>();
        for (int i = 0; i < tuple.size(); i++) {
            Integer digit = tuple.get(i);

            List<Integer> subTuple = new ArrayList<>(tuple);
            subTuple.remove(digit);
            List<List<Integer>> allVariants = pickAllVariants(subTuple);

            allVariants.stream().forEach(integers -> integers.add(0, digit));
            results.addAll(allVariants);
        }
        return results.stream().distinct().collect(Collectors.toList());
    }
}