Flatten an array of arbitrarily nested arrays of integers into a flat array of integers. e.g.[[1, [2, [3]]], 4] -> [1,2,3,4].

flatten_array.java

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

public class FlattenArray {
    static List<Integer> flatten(Object[] input) {
        if (input == null) return null;

        List<Integer> acc = new ArrayList<>();

        for (Object el : input) {
            if (el instanceof Integer) {
                acc.add((Integer) el);
            } else if (el instanceof Object[]) {
                acc.addAll(flatten((Object[]) el));
            } else {
                throw new IllegalArgumentException("Input must be an array of Integers or nested arrays of Integers");
            }
        }

        return acc;
    }

    public static void main(String[] args) {
        // [[1, [2, [3]]], 4]
        Object[] a3 = {3};
        Object[] a2 = {2, a3};
        Object[] a1 = {1, a2};
        Object[] input = {a1, 4};

        System.out.println(flatten(input));
    }
}