public List<Integer> grep(String main, String sub) {
        List<Integer> indicies = new ArrayList<>();

        for (int i = 0 ; i < main.length() ; i++) {
            char a = main.charAt(i);
            char b = sub.charAt(0);
            if (a == b && substringMatch(main, sub, i)) indicies.add(i);
        }

        return indicies;
    }

    boolean substringMatch(String main, String sub, int start) {
        for (int i = 1 ; i < sub.length() ; i++) {
            int mainIndex = start + i;
            if (mainIndex >= main.length() || main.charAt(mainIndex) != sub.charAt(i)) return false;
        }

        return true;
    }