java将list转为树形结构

1、通过转化成json封装数据

[
    {
        "name":"甘肃省",
        "pid":0,
        "id":1
    },
    {
        "name":"天水市",
        "pid":1,
        "id":2
    },
    {
        "name":"秦州区",
        "pid":2,
        "id":3
    },
    {
        "name":"北京市",
        "pid":0,
        "id":4
    },
    {
        "name":"昌平区",
        "pid":4,
        "id":5
    }
]

现需要是使用java将以上数据转为树形结构，转化后下的结构如下

[
    {
        "children":[
            {
                "children":[
                    {
                        "name":"秦州区",
                        "pid":2,
                        "id":3
                    }
                ],
                "name":"天水市",
                "pid":1,
                "id":2
            }
        ],
        "name":"甘肃省",
        "pid":0,
        "id":1
    },
    {
        "children":[
            {
                "name":"昌平区",
                "pid":4,
                "id":5
            }
        ],
        "name":"北京市",
        "pid":0,
        "id":4
    }
]

代码如下 

    import com.alibaba.fastjson.JSON;
    import com.alibaba.fastjson.JSONArray;
    import com.alibaba.fastjson.JSONObject;

    public static JSONArray listToTree(JSONArray arr, String id, String pid, String child) {
        JSONArray r = new JSONArray();
        JSONObject hash = new JSONObject();
        //将数组转为Object的形式，key为数组中的id
        for (int i = 0; i < arr.size(); i++) {
            JSONObject json = (JSONObject) arr.get(i);
            hash.put(json.getString(id), json);
        }
        //遍历结果集
        for (int j = 0; j < arr.size(); j++) {
            //单条记录
            JSONObject aVal = (JSONObject) arr.get(j);
            //在hash中取出key为单条记录中pid的值
            String pidStr = "";
            Object pidObj = aVal.get(pid);
            if (aVal.get(pid) != null) {
                pidStr = aVal.get(pid).toString();
            }
            JSONObject hashVP = (JSONObject) hash.get(pidStr);
            //如果记录的pid存在，则说明它有父节点，将她添加到孩子节点的集合中
            if (hashVP != null) {
                //检查是否有child属性
                if (hashVP.get(child) != null) {
                    JSONArray ch = (JSONArray) hashVP.get(child);
                    ch.add(aVal);
                    hashVP.put(child, ch);
                } else {
                    JSONArray ch = new JSONArray();
                    ch.add(aVal);
                    hashVP.put(child, ch);
                }
            } else {
                r.add(aVal);
            }
        }
        return r;
    }

public static void main(String[] args){
   List<Map<String,Object>> data = new ArrayList<>();
   Map<String,Object> map = new HashMap<>();
   map.put("id",1);
   map.put("pid",0);
   map.put("name","甘肃省");
   data.add(map);
   Map<String,Object> map2 = new HashMap<>();
   map2.put("id",2);
   map2.put("pid",1);
   map2.put("name","天水市");
   data.add(map2);
   Map<String,Object> map3 = new HashMap<>();
   map3.put("id",3);
   map3.put("pid",2);
   map3.put("name","秦州区");
   data.add(map3);
   Map<String,Object> map4 = new HashMap<>();
   map4.put("id",4);
   map4.put("pid",0);
   map4.put("name","北京市");
   data.add(map4);
   Map<String,Object> map5 = new HashMap<>();
   map5.put("id",5);
   map5.put("pid",4);
   map5.put("name","昌平区");
   data.add(map5);
   System.out.println(JSON.toJSONString(data));
   JSONArray result = 
   listToTree(JSONArray.parseArray(JSON.toJSONString(data)),"id","pid","children");
   System.out.println(JSON.toJSONString(result));
}