code: public String getDisplay(HashSet<String> s, String a, String b) {
          if (s.contains(a)) {
              s.add(b);
          } else {
              s.add(a + b);
          }
  
          StringBuilder sb = new StringBuilder();
          Iterator<String> it = s.iterator();
          while (it.hasNext()) {
              sb.append(it.next());
          }
          return sb.toString();
      }
docs: tag OCP, hashset could be Set instead