hash，以及数据结构——map容器

1. 实体类

public class Department{    
    private Long id;  //部门的id
    private String name;  //部门名字
​
    private Long parent_id;  //上级部门id
    private Department parent; //上级部门
​
    @JsonInclude(JsonInclude.Include.NON_EMPTY)//数据库不存在该字段
    private List<Department> children = new ArrayList<>();
}

2. 算法

public List<Department> tree() {
    //1.查询所有对象
    List<Department> all = departmentMapper.loadAll();
    //2.建立id-department关系
    HashMap<Long,Department> map = new HashMap<>();
    all.forEach(dept->{
        map.put(dept.getId(),dept);
    });
    //3.维护对象之间的关系，List里装所有的祖宗结点
    List<Department> list = new ArrayList<>();
    for(Department department : all){        //如果没有父节点，是祖宗
        if (department.getParent_id() == null){
            list.add(department);
            continue;
        }
        //如果department不是祖宗  说明有爹
        Long parent_id = department.getParent_id(); //儿子department 拿到他爹的id
        Department parent = map.get(parent_id); //根据parent_id找到自己的爹
        parent.getChildren().add(department); //department的爹parent  拿出口袋 把儿子装进去
    }
    return list;
}