ShortestSubstring.java

private int shortestSubstring(String s) { // "dabbcabcd", "bcaacbc", "bab"
        // for 'dabbcabcd' there is 4 unique letters, so shortest substring can be minimum 4 letter length. if not,
        // we need to increase that length and keep searching.
        List<String> letters = new ArrayList<>();
        for (char each : s.toCharArray()){
            String letter = String.valueOf(each);
            if (!letters.contains(letter)) {
                letters.add(letter);
            }
        }

        int minLength = letters.size();
        int maxLength = s.length();

        Log.d(TAG, "minLength: " + minLength);
        Log.d(TAG, "maxLength: " + maxLength);

        for (int currentLength = minLength; currentLength <= maxLength; currentLength++) {
            int substringSize = maxLength - currentLength + 1;
            for (int j = 0; j < substringSize; j++) {
                String substring = s.substring(j, j + currentLength);
                boolean isFound = isSubstringContainAllLetters(letters, substring);
                if (isFound) {
                    Log.d(TAG, substring + " is a substring that contains all the characters in s. Length: " + currentLength);
                    return currentLength;
                }
            }
        }
        return maxLength;
    }

    private boolean isSubstringContainAllLetters(List<String> letters, String substring) {
        for (String letter : letters) {
            if (!substring.contains(letter)) {
                return false;
            }
        }
        return true;
    }