public static int gemstones(List<String> arr) {
    List<String> uniqueChar = new ArrayList<>();
    int gemstones = 0;
        
    // For each string, add all characters in uniqueChar
    for (int i = 0; i < arr.size(); i++) {
        uniqueChar.addAll(Arrays.asList(arr.get(i).split("")));
    }
        
    // Removing duplicate characters
    uniqueChar = new ArrayList<>(new LinkedHashSet<>(uniqueChar));
        
    for (int i = 0; i < uniqueChar.size(); i++) {
        String c = uniqueChar.get(i);
        int counter = 0;
            
        for (int j = 0; j < arr.size(); j++) { 
            String str = arr.get(j);           
            counter += (str.indexOf(c) != -1) ? 1 : 0;
        }
            
        gemstones += (counter == arr.size()) ? 1 : 0;
    }
        
    return gemstones;
}