String matching in an array

class Solution {
    public List<String> stringMatching(String[] words) {
        if (words == null || words.length == 0) {
            return new ArrayList<>();
        }
        HashSet<String> set = new HashSet<>();
        for (String i : words) {
            if (!set.contains(i)) {
                for (String j : words) {
                    if (j.contains(i) && !j.equals(i)) {
                        set.add(i);
                        break;
                    }
                }
            }
        }
        return new ArrayList<>(set);
    }
}

or

StringBuilder builder = new StringBuilder();
        for (String word : words) {
            builder.append(word).append(",");
        }
        String wholeWords = builder.toString();
        List<String> resultList = new ArrayList<>();
        for (String word : words) {
            int index1 = wholeWords.indexOf(word);
            int index2 = wholeWords.lastIndexOf(word);
            if (index1 >= 0 && index2 >= 0 && index1 != index2) {
                resultList.add(word);
            }
        }
        return resultList;
        ```