docs: public static void printTree(Cluster a,String file) {  
  
  		int depth = getDepth(a);
  	    
  	    width =  wunit*(depth+1);
  		height = hunit*(depth+1);
  		BufferedImage image  = new BufferedImage(width, height, BufferedImage.TYPE_INT_ARGB);
  		Graphics2D g = image .createGraphics();
  		
  		g.setColor(new Color(0,0,0)); 
  		g.setStroke(new BasicStroke(1)); 
  		g.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
  		Font font = new Font("宋体", Font.BOLD, 20); 
  		
  		g.setFont(font);   
  		
  		drawTree(a, g, width/2, 0 , 1);
  		//释放对象 
  		g.dispose(); 
  		// 保存文件    
  		try {
  			ImageIO.write(image, "png", new File(file));
  		} catch (IOException e) {
  			// TODO Auto-generated catch block
  			e.printStackTrace();
  		} 
  	}