[Java知识库]JAVA 省市区对象 转成 树结构

例如从数据库中获取这种格式数据，然后要转成树结构

无需多重遍历，时间复杂度O(n)，用map来存放每个key的索引位置，然后根据key在树List中找到对应的层级，进行添加数据。
附上代码：

public class TestDemo {
    public static void main(String[] args) {
        Address address = new Address();
        address.setProvince("广东省");
        address.setCity("深圳市");
        address.setZone("宝安区");

        Address address2 = new Address();
        address2.setProvince("广东省");
        address2.setCity("广州市");
        address2.setZone("天河区");

        Address address3 = new Address();
        address3.setProvince("广东省");
        address3.setCity("广州市");
        address3.setZone("从化区");

        Address address4 = new Address();
        address4.setProvince("浙江省");
        address4.setCity("杭州市");
        address4.setZone("西湖区");
        List<Address> addressList = Arrays.asList(address, address2, address3, address4);

        // treeList 树结构 ； map存放索引位置
        // key 格式 省、省_市、省_市_区, 为了防止有市相同或者区相同，造成key值一致
        List<AdressTree> treeList = new ArrayList<>();
        Map<String, Integer> map = new HashMap<>();
        addressList.forEach(v->{
            String province = v.getProvince();
            Integer provinceIndex = map.get(province);
            // provinceIndex 不是 null，说明已在list里
            if (provinceIndex == null) {
                AdressTree tree = new AdressTree(province, province);
                treeList.add(tree);
                provinceIndex = treeList.size() - 1;
                map.put(province, provinceIndex);

            }

            String city = v.getCity();
            String cityKey = province + "_" + city;
            Integer cityIndex = map.get(cityKey);
            if (cityIndex == null) {
                List<AdressTree> children = treeList.get(provinceIndex).getChildren();
                AdressTree tree = new AdressTree(cityKey, city);
                children.add(tree);
                cityIndex = children.size() - 1;
                map.put(cityKey, cityIndex);
            }

            String zone = v.getZone();
            String zoneKey = cityKey + "_" + zone;
            Integer zoneIndex = map.get(zoneKey);
            if (zoneIndex == null) {
                List<AdressTree> children = treeList.get(provinceIndex).getChildren().get(cityIndex).getChildren();
                AdressTree tree = new AdressTree(zoneKey, zone);
                children.add(tree);
                zoneIndex = children.size() - 1;
                map.put(zoneKey, zoneIndex);
            }
        });

        System.out.println(JSONArray.toJSONString(treeList));
    }
}

@Data
class Address {
    private String province;
    private String city;
    private String zone;
}

@Data
class AdressTree {
    private String key;
    private String title;
    private List<AdressTree> children = new ArrayList<>();

    public AdressTree(String key, String title) {
        this.key = key;
        this.title = title;
    }
}

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