Java递归如何正确输出树形菜单

package com.tree; 
 
public class Node { 
 private Integer id; 
 private Integer parentId; 
 private String name; 
 private String link; 
  
 public Integer getId() { 
  return id; 
 } 
 public void setId(Integer id) { 
  this.id = id; 
 } 
 public Integer getParentId() { 
  return parentId; 
 } 
 public void setParentId(Integer parentId) { 
  this.parentId = parentId; 
 } 
 public String getName() { 
  return name; 
 } 
 public void setName(String name) { 
  this.name = name; 
 } 
 public String getLink() { 
  return link; 
 } 
 public void setLink(String link) { 
  this.link = link; 
 } 
} 

package com.tree; 
 
import java.util.ArrayList; 
import java.util.List; 
 
public class Tree { 
 private StringBuffer html = new StringBuffer(); 
 private List<Node> nodes; 
  
 public Tree(List<Node> nodes){ 
  this.nodes = nodes; 
 } 
  
 public String buildTree(){ 
  html.append("<ul>"); 
  for (Node node : nodes) { 
   Integer id = node.getId(); 
   if (node.getParentId() == null) { 
    html.append("\r\n<li id='" + id + "'>" + node.getName()+ "</li>"); 
    build(node); 
   } 
  } 
  html.append("\r\n</ul>"); 
  return html.toString(); 
 } 
  
 private void build(Node node){ 
  List<Node> children = getChildren(node); 
  if (!children.isEmpty()) { 
   html.append("\r\n<ul>"); 
   for (Node child : children) { 
    Integer id = child.getId(); 
    html.append("\r\n<li id='" + id + "'>" + child.getName()+ "</li>"); 
    build(child); 
   } 
   html.append("\r\n</ul>"); 
  }  
 } 
  
 private List<Node> getChildren(Node node){ 
  List<Node> children = new ArrayList<Node>(); 
  Integer id = node.getId(); 
  for (Node child : nodes) { 
   if (id.equals(child.getParentId())) { 
    children.add(child); 
   } 
  } 
  return children; 
 } 
} 

import java.util.ArrayList; 
import java.util.List; 
 
import cn.com.tree.Node; 
import cn.com.tree.Tree; 
 
 
public class Test { 
 
 /** 
  * @param args 
  */ 
 public static void main(String[] args) { 
  List<Node> nodes = new ArrayList<Node>(); 
   
  Node node1 = new Node(); 
  node1.setId(1); 
  node1.setName("node1"); 
  node1.setParentId(null); 
  node1.setLink(null); 
  nodes.add(node1); 
   
  Node node11 = new Node(); 
  node11.setId(11); 
  node11.setName("node11"); 
  node11.setParentId(1); 
  node11.setLink(null); 
  nodes.add(node11); 
   
  Node node111 = new Node(); 
  node111.setId(111); 
  node111.setName("node111"); 
  node111.setParentId(11); 
  node111.setLink(null); 
  nodes.add(node111); 
   
  Node node12 = new Node(); 
  node12.setId(12); 
  node12.setName("node12"); 
  node12.setParentId(1); 
  node12.setLink(null); 
  nodes.add(node12); 
   
  Node node2 = new Node(); 
  node2.setId(2); 
  node2.setName("node2"); 
  node2.setParentId(null); 
  node2.setLink(null); 
  nodes.add(node2); 
   
  Node node21 = new Node(); 
  node21.setId(21); 
  node21.setName("node21"); 
  node21.setParentId(2); 
  node21.setLink(null); 
  nodes.add(node21); 
   
  Node node3 = new Node(); 
  node3.setId(3); 
  node3.setName("node3"); 
  node3.setParentId(null); 
  node3.setLink(null); 
  nodes.add(node3); 
   
  Tree tree = new Tree(nodes); 
  System.out.println(tree.buildTree()); 
 } 
} 

<ul> 
<li id='1'>node1</li> 
<ul> 
<li id='11'>node11</li> 
<ul> 
<li id='111'>node111</li> 
</ul> 
<li id='12'>node12</li> 
</ul> 
<li id='2'>node2</li> 
<ul> 
<li id='21'>node21</li> 
</ul> 
<li id='3'>node3</li> 
</ul>