code: @Override
    protected int computeHeight() {
      N node = getRoot();
      int tHeight = 1;
  
      // compute height
      while(!node.isLeaf() && node.getNumEntries() != 0) {
        SpatialEntry entry = node.getEntry(0);
        node = getNode(entry);
        tHeight++;
      }
      return tHeight;
    }
docs: Computes the height of this XTree. Is called by the constructor. and should
  be overwritten by subclasses if necessary.
  
  @return the height of this XTree