Well-ordered String

Problem

You know a password is well-ordered string. Well-ordered string means that the order of the characters is in an alphabetical increasing order. Like “abEm” is a well-ordered number. However, “abmE” is not a well-order number. Given an input# that tells you also how many digits are in the password, print all possible 

Solution

Recursive + DFS

    public static List<List<String>> hackCode(int num) {
        List<List<String>> res = new ArrayList<List<String>>();
        if (num < 0) {
            return res;
        }

        List<String> ls = new ArrayList<String>();
        helper(res, ls, 0, num, 0);
        return res;
    }

    public static void helper(List<List<String>> res, List<String> ls, int prev,
            int num, int pos) {
        if (pos == num) {
            res.add(new ArrayList<String>(ls));
            return;
        }

        for (int i = prev; i < 26; i++) {
            char c = (char) ('a' + i);
            ls.add(String.valueOf(c));
            helper(res, ls, i + 1, num, pos + 1);
            ls.remove(ls.size() - 1);
        }
    }