code: public HashSet<String> getAllStrings()
      {
          HashSet<String> strHashSet = new LinkedHashSet<String>();
  
          if (sourceNode != null)
              getStrings(strHashSet, SearchCondition.NO_SEARCH_CONDITION, null, "", sourceNode.getOutgoingTransitions());
          else
              getStrings(strHashSet, SearchCondition.NO_SEARCH_CONDITION, null, "", simplifiedSourceNode);
  
          return strHashSet;
      }
docs: 取出所有key<br>
  Retrieves all the valid Strings that have been inserted in to the MDAG.
  
  @return a HashSet containing all the Strings that have been inserted into the MDAG