Permutation algorithm for array of integers in Java

Question

I have a working example to generate all char permutations in a String as below:

static ArrayList permutations(String s) {
        if (s == nul         


        

Answer 1

Here is my solution (gist):

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.function.Consumer;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

/**
 * @author Karol Krol
 */
public class Permutation {

    private Permutation() {
    }

    public static List> permutation(final int[] numbers) {
        final PermutationCollector permutationCollector = new PermutationCollector();
        permutation(new int[0], numbers, permutationCollector);
        return permutationCollector.getResult();
    }

    private static void permutation(int[] prefix, int[] array, final Consumer operation) {
        int length = array.length;
        if (length == 0) {
            operation.accept(prefix);
        } else {
            for (int i = 0; i < length; ++i) {
                final int[] newPrefix = append(prefix, array[i]);
                final int[] reducedArray = reduce(array, i);
                permutation(newPrefix, reducedArray, operation);
            }
        }
    }

    private static int[] append(int[] array, int element) {
        int newLength = array.length + 1;
        array = Arrays.copyOf(array, newLength);
        array[newLength - 1] = element;
        return array;
    }

    private static int[] reduce(int[] array, int index) {
        final int newLength = array.length - 1;
        if (index == 0) {
            return Arrays.copyOfRange(array, 1, array.length);
        } else {
            final int[] dest = new int[newLength];
            System.arraycopy(array, 0, dest, 0, index);
            System.arraycopy(array, index + 1, dest, index, newLength - index);
            return dest;
        }
    }

    public static class PermutationCollector implements Consumer {

        private List> result = new ArrayList<>();

        @Override
        public void accept(int[] ints) {
            result.add(IntStream.of(ints).boxed().collect(Collectors.toList()));
        }

        public List> getResult() {
            return result;
        }
    }
}