public void addNode(Node newNode){  
         int comp = newNode.data.compareTo(data);
         if (comp < 0){  
            if (left == null){
               left = newNode;
               left.parent = this;
            }
            else { left.addNode(newNode); }
         }
         else if (comp > 0){  
            if (right == null){
               right = newNode;
               right.parent = this;
            }
            else { right.addNode(newNode); }
         }
      }