List转换为tree-项目真实使用

前提：数据库中查出来每一条数据构成链表，需要我们转换成树结构，id，pid（父节点的id） name

主要代码：public Node getRoot(List<Node> list)
0为根节点 1-9为次根节点 1.1-1.9 2.1-2.9 … 为次次根节点 看下方测试结果注意只有0的pid为-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","根节点"));
        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")) {
     //根结点的判断
                root = node;
                continue;
            }
            Node node1 = map.get(pid);
            if(node1==null){
    
                node1 = new Node(pid,"-1","头节点");
                map.put(pid,node1);
            }
            node1.getChildren().add(node);
        }
        return  root;
    }
}

测试结果

{
    
  "success": true,
  "message": "操作成功！",
  "code": 0,
  "result": {
    
    "id": "0",
    "pid": "-1",
    "name": "根节点",
    "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
}

解释：

1. 每次先查看map中是不是有id对应的key，如果有的话，说明之前添加过但是添加的对象只有id和children，这时候就需要把对应value中的对象的其他变量赋值，这个为什么只有id一会看到下边就知道了。
   如果没有对应的key是id的话，添加key为id ，value为Node对象。
2. 1中添加完id对应的Node之后，如果当前id节点的pid节点已存在，则把当前id节点添加到pid节点children数组中。如果不存在，需要先创建一个pid的新节点，并把新节点的id=pid，新节点添加到map中，并把1中的Node节点添加到新结点的children中（知道1中那个为什么只有id和children了吧）。