Permutation of array

Question

For example I have this array:

int a[] = new int[]{3,4,6,2,1};

I need list of all permutations such that if one is like this, {3,2,1

Answer 1

Implementation via recursion (dynamic programming), in Java, with test case (TestNG).

---

Code

PrintPermutation.java

import java.util.Arrays;

/**
 * Print permutation of n elements.
 * 
 * @author eric
 * @date Oct 13, 2018 12:28:10 PM
 */
public class PrintPermutation {
    /**
     * Print permutation of array elements.
     * 
     * @param arr
     * @return count of permutation,
     */
    public static int permutation(int arr[]) {
        return permutation(arr, 0);
    }

    /**
     * Print permutation of part of array elements.
     * 
     * @param arr
     * @param n
     *            start index in array,
     * @return count of permutation,
     */
    private static int permutation(int arr[], int n) {
        int counter = 0;
        for (int i = n; i < arr.length; i++) {
            swapArrEle(arr, i, n);
            counter += permutation(arr, n + 1);
            swapArrEle(arr, n, i);
        }
        if (n == arr.length - 1) {
            counter++;
            System.out.println(Arrays.toString(arr));
        }

        return counter;
    }

    /**
     * swap 2 elements in array,
     * 
     * @param arr
     * @param i
     * @param k
     */
    private static void swapArrEle(int arr[], int i, int k) {
        int tmp = arr[i];
        arr[i] = arr[k];
        arr[k] = tmp;
    }
}

PrintPermutationTest.java (test case via TestNG)

import org.testng.Assert;
import org.testng.annotations.Test;

/**
 * PrintPermutation test.
 * 
 * @author eric
 * @date Oct 14, 2018 3:02:23 AM
 */
public class PrintPermutationTest {
    @Test
    public void test() {
        int arr[] = new int[] { 0, 1, 2, 3 };
        Assert.assertEquals(PrintPermutation.permutation(arr), 24);

        int arrSingle[] = new int[] { 0 };
        Assert.assertEquals(PrintPermutation.permutation(arrSingle), 1);

        int arrEmpty[] = new int[] {};
        Assert.assertEquals(PrintPermutation.permutation(arrEmpty), 0);
    }
}