private void permuteHelper(int[] num, int left) {
        if (left == num.length - 1) {
            ArrayList<Integer> list = new ArrayList<Integer>();
            for (int i = 0; i < num.length; i++) {
                list.add(num[i]);
            }
            if (set.add(list))
                result.add(list);
            return;
        }

        for (int right = left; right < num.length; right++) {
            swap(num, left, right);
            permuteHelper(num, left + 1);
            swap(num, left, right);
        }
    }