java后端递归树，机构树，遍历树

   public static void getChildrenList(List<JSONObject> list,JSONObject pJo){
        List<JSONObject> retList=new ArrayList<JSONObject>();
        for(JSONObject jo:list){
            if(jo.getString("pid").equals(pJo.getString("id"))){
                retList.add(jo);
                jo.put("children", getChildrenList(list, jo));
            }
        }
        //return retList;
        pJo.put("children",retList);
    }
    
    

第一种，方法循环一次，比较耗费内存，不建议使用

public static void main(String[] args) {

                //拼装数据
        List<JSONObject> list=new ArrayList<JSONObject>();
        list.add(JSONObject.parseObject("{id:'1',name:'n1',pid:'-1'}"));
        list.add(JSONObject.parseObject("{id:'2',name:'n1',pid:'-1'}"));
        list.add(JSONObject.parseObject("{id:'3',name:'n1',pid:'-1'}"));
        
        list.add(JSONObject.parseObject("{id:'1-1',name:'n1-1',pid:'1'}"));
        list.add(JSONObject.parseObject("{id:'2-1',name:'n2-1',pid:'2'}"));
        list.add(JSONObject.parseObject("{id:'3-1',name:'n3-1',pid:'3'}"));
        list.add(JSONObject.parseObject("{id:'1-2',name:'n1-2',pid:'1'}"));
        list.add(JSONObject.parseObject("{id:'2-2',name:'n2-2',pid:'2'}"));
        list.add(JSONObject.parseObject("{id:'3-2',name:'n3-2',pid:'3'}"));
        list.add(JSONObject.parseObject("{id:'1-3',name:'n1-3',pid:'1'}"));
        list.add(JSONObject.parseObject("{id:'2-3',name:'n2-3',pid:'2'}"));
        list.add(JSONObject.parseObject("{id:'3-3',name:'n3-3',pid:'3'}"));
        
        list.add(JSONObject.parseObject("{id:'1-1-1',name:'n1-1-1',pid:'1-1'}"));
        list.add(JSONObject.parseObject("{id:'1-1-2',name:'n1-1-2',pid:'1-1'}"));
        list.add(JSONObject.parseObject("{id:'1-1-3',name:'n1-1-3',pid:'1-1'}"));        
        
        //递归  根节点
        JSONObject tJo=JSONObject.parseObject("{id:'-1',name:'根节点'}");
        //tJo.put("children",getChildrenList(list, tJo));
        System.out.println(tJo.toJSONString());
        
        
        //一次循环 方法，比较好内存不建议使用
        Map<String,List<JSONObject>> map1=new HashMap<String, List<JSONObject>>();
        for(JSONObject jo:list){
            String pid=jo.getString("pid");
            String id=jo.getString("id");
            if(!map1.containsKey(pid)){//不存在
                map1.put(pid, new ArrayList<JSONObject>());
            }
            map1.get(pid).add(jo);
            
            if(!map1.containsKey(id)){
                map1.put(id, new ArrayList<JSONObject>());
            }
            jo.put("children", map1.get(id));
        }
        JSONObject tJo2=JSONObject.parseObject("{id:'-1',name:'根节点'}");
        tJo2.put("children", map1.get("-1"));
        System.out.println(tJo2.toJSONString());
        
    }
    
    

第二种方法，两次循环，建议使用，节省内存

public static void main(String[] args) {

                //拼装数据
        List<JSONObject> list=new ArrayList<JSONObject>();
        list.add(JSONObject.parseObject("{id:'1',name:'n1',pid:'-1'}"));
        list.add(JSONObject.parseObject("{id:'2',name:'n1',pid:'-1'}"));
        list.add(JSONObject.parseObject("{id:'3',name:'n1',pid:'-1'}"));
        
        list.add(JSONObject.parseObject("{id:'1-1',name:'n1-1',pid:'1'}"));
        list.add(JSONObject.parseObject("{id:'2-1',name:'n2-1',pid:'2'}"));
        list.add(JSONObject.parseObject("{id:'3-1',name:'n3-1',pid:'3'}"));
        list.add(JSONObject.parseObject("{id:'1-2',name:'n1-2',pid:'1'}"));
        list.add(JSONObject.parseObject("{id:'2-2',name:'n2-2',pid:'2'}"));
        list.add(JSONObject.parseObject("{id:'3-2',name:'n3-2',pid:'3'}"));
        list.add(JSONObject.parseObject("{id:'1-3',name:'n1-3',pid:'1'}"));
        list.add(JSONObject.parseObject("{id:'2-3',name:'n2-3',pid:'2'}"));
        list.add(JSONObject.parseObject("{id:'3-3',name:'n3-3',pid:'3'}"));
        
        list.add(JSONObject.parseObject("{id:'1-1-1',name:'n1-1-1',pid:'1-1'}"));
        list.add(JSONObject.parseObject("{id:'1-1-2',name:'n1-1-2',pid:'1-1'}"));
        list.add(JSONObject.parseObject("{id:'1-1-3',name:'n1-1-3',pid:'1-1'}"));
        
        
        //递归
        JSONObject tJo=JSONObject.parseObject("{id:'-1',name:'根节点'}");
        //tJo.put("children",getChildrenList(list, tJo));
        System.out.println(tJo.toJSONString());
        
        
        
        
        
        
        //两次循环 方法，建议使用
        Map<String,List<JSONObject>> map=new HashMap<String, List<JSONObject>>();
        for(JSONObject jo:list){
            String pid=jo.getString("pid");
            if(!map.containsKey(pid)){//不存在
                map.put(pid, new ArrayList<JSONObject>());
            }
            map.get(pid).add(jo);
        }
        
        for(JSONObject jo:list){
            String id=jo.getString("id");
            if(map.containsKey(id)){
                jo.put("children", map.get(id));
            }
        }
        JSONObject tJo1=JSONObject.parseObject("{id:'-1',name:'根节点'}");
        tJo1.put("children", map.get("-1"));
        System.out.println(tJo1.toJSONString());
        
        }