java如何用递归生成树形结构

public class TreeNode{
	private int id;
	private int pid;
	private String name;
	private String content;
	private List<TreeNode> children;

	public TreeNode() {
	}

	public TreeNode(int id, int pid, String name, String content) {
		this.id = id;
		this.pid = pid;
		this.name = name;
		this.content = content;
	}

	public int getId() {
		return id;
	}

	public void setId(int id) {
		this.id = id;
	}

	public int getPid() {
		return pid;
	}

	public void setPid(int pid) {
		this.pid = pid;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public String getContent() {
		return content;
	}

	public void setContent(String content) {
		this.content = content;
	}

	public List<TreeNode> getChildren(){
		return children;
	}

	public void setChildren(List<TreeNode> children){
		this.children = children;
	}

	@Override
	public String toString() {
		return "TreeNode{" +
				"id=" + id +
				", pid=" + pid +
				", name='" + name + '\'' +
				", content='" + content + '\'' +
				'}';
	}
}

//生成测试数据方法
public static List<TreeNode> generateList(){
	List<TreeNode> list = new ArrayList<>();
	TreeNode treeModel1 = new TreeNode(1,0,"系统管理",getRandomStr());
	TreeNode treeModel2 = new TreeNode(11,1,"用户管理",getRandomStr());
	TreeNode treeModel3 = new TreeNode(111,11,"用户添加",getRandomStr());
	TreeNode treeModel4 = new TreeNode(2,0,"权限管理",getRandomStr());
	TreeNode treeModel5 = new TreeNode(21,2,"账号管理",getRandomStr());
	TreeNode treeModel6 = new TreeNode(221,2,"角色管理",getRandomStr());
	list.add(treeModel1);
	list.add(treeModel2);
	list.add(treeModel3);
	list.add(treeModel4);
	list.add(treeModel5);
	list.add(treeModel6);
	return list;
}

//随便写了一个由26个小写字母随机生成字符串方法
public static String getRandomStr(){
	StringBuilder ret= new StringBuilder();
	for (int i = 0; i < 6; i++) {
		int num=(int)(Math.random()*26+97);
		ret.append((char) num);
	}
	return ret.toString();
}

/**
 * 生成树
 * @param list 待处理的数据集合
 * @param pid 最顶层的父级id
 */
public static List<TreeNode> createTree(List<TreeNode> list,int pid){
	//根据父id过滤出所有父节点
	List<TreeNode> tree = list.stream().filter(e-> pid == e.getPid()).collect(Collectors.toList());
	//遍历所有父节点，把属于该节点的所有子节点加到该节点的children属性下
	for (TreeNode node: tree) {
		int id = node.getId();
		//过滤出所有属于该节点的子节点
		List<TreeNode> cList = list.stream().filter(e-> id == e.getPid()).collect(Collectors.toList());
		//判断是否存在子节点，若子节点有数据则把该子节点的id作为pid继续递归往下层处理
		if(!SysUtil.isNull(cList)){
			createTree(list,id);
			node.setChildren(cList);
		}
	}
	return tree;
}

public static void main(String[] args) throws Exception {

	List<TreeNode> list = generateList();
	list.forEach(System.out::println);
	List<TreeNode> tree = createTree(list,0);
	System.out.println(JSON.toJSONString(tree));


}