List converted to tree real use of the project

Premise ： Each piece of data found in the database constitutes a linked list , We need to transform it into a tree structure ,id,pid（ Parent node id） name

Main code ：public Node getRoot(List<Node> list)
0 Root node 1-9 Is a secondary root node 1.1-1.9 2.1-2.9 … It is a secondary root node See the test results below Note that only 0 Of pid by -1

@RestController
public class AAA {
    
    @RequestMapping("/project/test")
    public Result get123(){
    
        Test test = new Test();
        List<Node> list = new ArrayList<>();
        list.add(new Node("0","-1"," The root node "));
        list.add(new Node("9","0","9"));
        list.add(new Node("9.1","9","9.1"));

        list.add(new Node("1","0","1"));
        list.add(new Node("1.1","1","1.1"));

        list.add(new Node("2","0","2"));
        list.add(new Node("2.1","2","2.1"));

        list.add(new Node("3","0","3"));
        list.add(new Node("3.1","3","3.1"));
        list.add(new Node("3.1.2","3.1","3.1.2"));
        Node root = test.getRoot(list);
        Result<Object> result = new Result<>();
        System.out.println(root);
        result.setResult(root);
        return result;
    }
}
class Node{
    
    String id;
    String pid;
    String name;
    List<Node> children;
    public Node(String id,String pid,String name){
    
        this.id=id;
        this.pid = pid;
        this.name = name;
        this.children = new ArrayList<>();
    }

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

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

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

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

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

    public String getId() {
    
        return id;
    }

    public String getName() {
    
        return name;
    }

    public String getPid() {
    
        return pid;
    }
}

class Test{
    

    public Node getRoot(List<Node> list){
    
        Node root=null;
        Map<String,Node> map= new HashMap<String,Node>();
        for(Node n:list){
    
            String id = n.getId();
            Node node = map.get(id);
            if(node==null){
    
                node = n;
                map.put(id,n);
            }else{
    
                node = map.get(id);
                node.setPid(n.getPid());
                node.setName(n.getName());
            }
            String pid = node.getPid();
            if(pid.equals("-1")) {
     // Judgment of root node 
                root = node;
                continue;
            }
            Node node1 = map.get(pid);
            if(node1==null){
    
                node1 = new Node(pid,"-1"," Head node ");
                map.put(pid,node1);
            }
            node1.getChildren().add(node);
        }
        return  root;
    }
}

test result

{
    
  "success": true,
  "message": " Successful operation ！",
  "code": 0,
  "result": {
    
    "id": "0",
    "pid": "-1",
    "name": " The root node ",
    "children": [
      {
    
        "id": "9",
        "pid": "0",
        "name": "9",
        "children": [
          {
    
            "id": "9.1",
            "pid": "9",
            "name": "9.1",
            "children": [
              
            ]
          }
        ]
      },
      {
    
        "id": "1",
        "pid": "0",
        "name": "1",
        "children": [
          {
    
            "id": "1.1",
            "pid": "1",
            "name": "1.1",
            "children": [
              
            ]
          }
        ]
      },
      {
    
        "id": "2",
        "pid": "0",
        "name": "2",
        "children": [
          {
    
            "id": "2.1",
            "pid": "2",
            "name": "2.1",
            "children": [
              
            ]
          }
        ]
      },
      {
    
        "id": "3",
        "pid": "0",
        "name": "3",
        "children": [
          {
    
            "id": "3.1",
            "pid": "3",
            "name": "3.1",
            "children": [
              {
    
                "id": "3.1.2",
                "pid": "3.1",
                "name": "3.1.2",
                "children": [
                  
                ]
              }
            ]
          }
        ]
      }
    ]
  },
  "timestamp": 1651161878529
}

explain ：

1. Check first every time map Is there any id Corresponding key, If any , Note that the objects added before are only id and children, At this time, we need to correspond value Assignment of other variables of the object in , Why is there only id I'll see it later .
   If there is no counterpart key yes id Words , add to key by id ,value by Node object .
2. 1 After adding id Corresponding Node after , If at present id Node pid Node already exists , Put the current id Add nodes to pid node children Array . If it doesn't exist , I need to create one first pid The new node , And put the new node id=pid, New nodes are added to map in , And put 1 Medium Node The node is added to the new node children in （ know 1 Why is there only id and children Is that right ）.