question: private void buildString ( StringBuilder sb , TwoDNode node ) { if ( node == null ) { return ; } TwoDNode left = node . getBelow ( ) ; TwoDNode right = node . getAbove ( ) ; if ( left != null ) { buildString ( sb , left ) ; } sb . append ( node . toString ( ) ) ; if ( right != null ) { buildString ( sb , right ) ; } }
target:  Helper method for toString(). 