private static boolean isUniqueSymbols(
      final String string) {
    if (string == null || string.length() > 256) {
      return false;
    }

    boolean[] chars = new boolean[256];
    for (int i = 0; i < string.length(); i++) {
      int pos = string.charAt(i);
      if (chars[pos]) {
        return false;
      }
      chars[pos] = true;

    }
    return true;
  }