public ArrayList<ArrayList<Integer>> permute(int[] num) {
    ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();

    //start from an empty list
    result.add(new ArrayList<Integer>());

    for (int i = 0; i < num.length; i++) {
        //list of list in current iteration of the array num
        ArrayList<ArrayList<Integer>> current = new ArrayList<ArrayList<Integer>>();

        for (ArrayList<Integer> l : result) {
            // # of locations to insert is largest index + 1
            for (int j = 0; j < l.size()+1; j++) {
                // + add num[i] to different locations
                l.add(j, num[i]);

                ArrayList<Integer> temp = new ArrayList<Integer>(l);
                current.add(temp);

                //System.out.println(temp);

                // - remove num[i] add
                l.remove(j);
            }
        }

        result = new ArrayList<ArrayList<Integer>>(current);
    }

    return result;
}