@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;
    }
}

public class TreeUtils {

    //把一個List轉成樹
    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,"中國",-1);
        Node node1=new Node(1,"湖北省",0);
        Node node2=new Node(2,"武漢市",1);
        Node node3=new Node(3,"洪山區(qū)",2);
        Node node4=new Node(4,"宜昌市",1);
        Node node5=new Node(5,"上海市",0);
        Node node6=new Node(6,"靜安區(qū)",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));
    }
}