private static List<String> toStr(char[] arr, Character prefix) {
    return IntStream.range(0, arr.length)
        .mapToObj(i -> {
          String v = String.valueOf(arr[i]);
          return prefix != null ? prefix + v : v;
        })
        .collect(Collectors.toList());
  }
private static List<String> toStr(char[] arr, Character prefix)