App.java

package codeTest;

import java.util.ArrayList;

public class App
{
    public static void main( String[] args ) {
        test2DArray();
        test3DArray();
    }

    /**
     * Recursively go through an array of integers and copy them to a resulting array,
     * if it contains nested arrays, add its integers to the target as well.
     *
     * @param array an ArrayList of integers and other ArrayLists
     * @param target the resulting array list of integers
     */
    public static void recursiveFlatten(Object array, ArrayList<Integer> target) {
        if (array instanceof ArrayList) {
            for (Object item : (ArrayList) array) {
                if (item instanceof ArrayList) {
                    recursiveFlatten(item, target);
                } else if (item instanceof Integer) {
                    target.add((Integer) item);
                }
            }
        }
    }

    /**
     * Test with an arraylist that contains integers and a one-level nested arraylist
     */
    public static void test2DArray() {
        ArrayList<Object> one = new ArrayList<>();
        ArrayList<Object> two = new ArrayList<>();
        ArrayList<Integer> flatten = new ArrayList<>();

        two.add(new Integer(10));
        two.add(new Integer(11));
        two.add(new Integer(12));

        one.add(new Integer(1));
        one.add(new Integer(2));
        one.add(two);
        one.add(new Integer(3));

        recursiveFlatten(one, flatten);
        System.out.println(one);
        System.out.println(flatten);
    }

    /**
     * Test with an arraylist that contains integers and a two-level nested arraylist
     */
    public static void test3DArray() {
        ArrayList<Object> one = new ArrayList<>();
        ArrayList<Object> two = new ArrayList<>();
        ArrayList<Object> three = new ArrayList<>();
        ArrayList<Integer> flatten = new ArrayList<>();

        three.add(new Integer(20));
        three.add(new Integer(21));
        three.add(new Integer(22));

        two.add(new Integer(10));
        two.add(three);
        two.add(new Integer(11));
        two.add(new Integer(12));

        one.add(new Integer(1));
        one.add(new Integer(2));
        one.add(two);
        one.add(new Integer(3));

        recursiveFlatten(one, flatten);
        System.out.println(one);
        System.out.println(flatten);
    }
}

This is how the result looks like on the console:

[1, 2, [10, 11, 12], 3]
[1, 2, 10, 11, 12, 3]
[1, 2, [10, [20, 21, 22], 11, 12], 3]
[1, 2, 10, 20, 21, 22, 11, 12, 3]