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>