ConcatInteger

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.*;
import java.util.stream.Collectors;

public class ConatCombinations {
    public static void main (String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        int[] aa = {1, 2, 3, 4};

        List<Integer> proba =
        Arrays.stream(aa)
                .boxed()
                .collect(Collectors.toList());
        proba.forEach(System.out::print);
        var res = concatComb(new HashSet<>(), proba, 0);
        res.forEach(System.out::print);
    }

    static HashSet<List<Integer>> concatComb(HashSet<List<Integer>> result,
                                             List<Integer> inp, int index) {
        if (index == inp.size()) {
            List<Integer> next = new LinkedList<>();
            next.addAll(inp);
            result.add(next);
        }
        for (int i = index; i < inp.size(); i++) {
            concatComb(result, inp, ++index);
            index--;
            int first = inp.get(i);
            int second = inp.get(i + 1);
            inp.set(i + 1, first * 10 + second);
            inp.remove(i);
            concatComb(result, inp, index);
            inp.set(i, second);
            inp.add(i, first);
        }
        return result;
    }
}