/**
  * @param BinaryNode $node
  * @param BinaryNode $subtree
  */
 public function insertNode(BinaryNode $node, BinaryNode &$subtree = null)
 {
     if ($subtree === null) {
         $subtree = $node;
     } else {
         if ($node->getKeySum() >= $subtree->getKeySum()) {
             $this->insertNode($node, $subtree->rightNode);
         } else {
             $this->insertNode($node, $subtree->leftNode);
         }
     }
 }