Permute2

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package einsteinsriddle;

import java.util.ArrayList;
import java.util.List;

/**
 * @author ??? && Joel Brito
 */
public class Permute2 {

    private static Object[] input;
    private static int outputIndex;
    private static List<List<Object>> output;

    public Permute2() {
        outputIndex = 0;
    }

    public List<List<Object>> getPermutation(Object[] input) {
        this.input = input;
        this.output = new ArrayList<>();

        permutations(0);
        return output;
    }

    private static void permutations(int offset) {
        if (input.length - offset == 1) {
// Input now contains a permutation, here I store it in output,
// but you can do anything you like with it

            List<Object> tmp = new ArrayList<>();
            for (Object o : input) {
                tmp.add(o);
            }
            output.add(tmp);
            return;
        }

        Object a = input[offset];

        for (int i = offset; i < input.length; i++) {
// Swap elements
            Object b = input[i];
            input[i] = a;
            input[offset] = b;

            permutations(offset + 1);

// Restore element
            input[i] = b;
        }

        input[offset] = a;
    }

    private static int factorial(int n) {
        return n == 1 ? n : n * factorial(n - 1);
    }
}