code: public static Iterator<Character> notExistChars(String s) {
      return new Iterator<Character>() {
        IntList collect = IntList.create(s.chars().distinct().sorted().toArray());
        int current = 128;
        Character next = null;
  
        @Override
        public Character next() {
          if (calcNext()) {
            Character c = next;
            next = null;
            return c;
          } else {
            throw new NoSuchElementException("No next non-exist character.");
          }
        }
  
        @Override
        public boolean hasNext() {
          return calcNext();
        }
  
        private boolean calcNext() {
          if (next == null) {
            while (++current < Integer.MAX_VALUE) {
              if (collect.remove(current) == false) {
                next = (char) current;
                return true;
              }
            }
            return false;
          }
          return true;
        }
      };
    }
docs: Get a list of not exist chars in given string. You can use these chars as placeholder to
  replace string safety.
  
  @param s
  @return