code: public HashSet<String> getStringsWithSubstring(String str)
      {
          HashSet<String> strHashSet = new HashSet<String>();
  
          if (sourceNode != null)      //if the MDAG hasn't been simplified
              getStrings(strHashSet, SearchCondition.SUBSTRING_SEARCH_CONDITION, str, "", sourceNode.getOutgoingTransitions());
          else
              getStrings(strHashSet, SearchCondition.SUBSTRING_SEARCH_CONDITION, str, "", simplifiedSourceNode);
  
          return strHashSet;
      }
docs: 返回包含字串的key<br>
  Retrieves all the Strings in the MDAG that contain a given String.
  
  @param str a String that is contained in all the desired Strings
  @return a HashSet containing all the Strings present in the MDAG that begin with {@code prefixString}