code: public String getDeepestFolderId()
      {
          if ( filesChain.isEmpty() ) {
              return "root";
          }
          JSONObject last = filesChain.getLast();
          if ( GoogleDrive.MIME_TYPE_DIRECTORY.equals( last.getString( "mimeType" ) ) ) {
              return last.getString( "id" );
          }
          // If last is a blob, we return the parent id :
          if ( filesChain.size() == 1 ) {
              return "root";
          }
          return filesChain.get( filesChain.size() - 2 ).getString( "id" );
      }
docs: If this remote path does not exist, this is the last existing id, or 'root'.
  
  @return id of deepest folder in filesChain, or "root" if filesChain is empty
  @throws IllegalStateException if last segment is a blob