public Object key(){
  StringBuilder key=new StringBuilder();
  for (int i=0; i < filled.length; i++) {
    if (filled[i]) {
      key.append('T');
    }
 else {
      key.append('F');
    }
  }
  return key.toString();
}

public Object key ( ) { StringBuilder key = new StringBuilder ( ) ; for ( int i = 0 ; i < filled . length ; i ++ ) { if ( filled [ i ] ) { key . append ( 'T' ) ; } else { key . append ( 'F' ) ; } } return key . toString ( ) ; }
suitable unique key for this node .
