在开发中，我们会遇到将不同组织架构合并成tree这种树状结构，那么如果做呢？
实际上，我们也可以理解为如何将拥有父子关系的list转成树形结构，而这其中主要的方法就是递归！

1、实体对象：

@Data
public class Node {
    private Integer id;
    private String city;
    private Integer pid;

    private List<Node> children;

    public Node(Integer id,String city,Integer pid){
        this.id = id;
        this.city = city;
        this.pid = pid;
    }
}

2、转换工具类：

public class TreeUtils {

    //把一个List转成树
    static List<Node> buildTree(List<Node> list,Integer pid){
        List<Node> tree=new ArrayList<>();
        for(Node node:list){
            if(Objects.equals(node.getPid(),pid)){
                tree.add(findChild(node,list));
            }
        }
        return tree;
    }

    static Node findChild(Node node, List<Node> list){
        for(Node n:list){
            if(Objects.equals(n.getPid(),node.getId())){
                if(node.getChildren() == null){
                    node.setChildren(new ArrayList<Node>());
                }
                node.getChildren().add(findChild(n,list));
            }
        }
        return node;
    }

    public static void main(String[] args) {
        Node node0=new Node(0,"中国",-1);
        Node node1=new Node(1,"湖北省",0);
        Node node2=new Node(2,"武汉市",1);
        Node node3=new Node(3,"洪山区",2);
        Node node4=new Node(4,"宜昌市",1);
        Node node5=new Node(5,"上海市",0);
        Node node6=new Node(6,"静安区",5);
        List<Node> list=new ArrayList<>();


        list.add(node3);
        list.add(node4);
        list.add(node1);
        list.add(node2);
        list.add(node5);
        list.add(node6);
        list.add(node0);
        List<Node> nodes = buildTree(list,-1);
        System.out.println(JSON.toJSONString(nodes));
    }
}

3、运行结果：

这样list就成功转换成为了tree装结构