java 上下級組裝，性能問題

需求，省市上下兩級，組裝成功一個json對象： 常規做法一：雙重for循環，如果省有10個，市有10個，那麼時間複雜度是100

 //省級 直轄市
        List<BaseRegion> provincialList = regionDao.getProvince();
        //市級，區級
        List<BaseRegion> municipalList = regionDao.getCity();

        for (BaseRegion region : provincialList) {
            //添加省級 直轄市
            RegionResDto regionResDto = new RegionResDto();
            regionResDto.setId(region.getId());
            regionResDto.setCode(region.getCode());
            regionResDto.setName(region.getName());

            List<RegionResDto> municipal = new ArrayList<>();
            for (BaseRegion baseRegion : municipalList) {
                //添加市級，區級
                if (baseRegion.getParentId().equals(region.getId())) {
                    RegionResDto childResDto = new RegionResDto();
                    childResDto.setId(baseRegion.getId());
                    childResDto.setCode(baseRegion.getCode());
                    childResDto.setName(baseRegion.getName());
                    municipal.add(childResDto);
                    regionResDto.setChildren(municipal);
                }
            }

接下來另一種思路，第一次遍歷，就把市級子類集合，存入map中，父id爲key，同樣省和市都爲10的話，複雜度爲20；

 

       //省級 直轄市
        List<BaseRegion> provincialList = regionDao.getProvince();
        //市級，區級
        List<BaseRegion> municipalList = regionDao.getCity();

        Map<Long, List<RegionResDto>> childListMap = new HashMap<>();

        for (BaseRegion baseRegion : municipalList) {
            List<RegionResDto> regionList = childListMap.get(baseRegion.getParentId());
            if (regionList == null) {
                regionList = new ArrayList<>();
            }
            RegionResDto childResDto = new RegionResDto();
            BeanUtils.copyProperties(baseRegion, childResDto);
            regionList.add(childResDto);
            childListMap.put(baseRegion.getParentId(), regionList);
        }

        for (BaseRegion region : provincialList) {
            //添加省級 直轄市
            RegionResDto regionResDto = new RegionResDto();
            BeanUtils.copyProperties(region, regionResDto);
            List<RegionResDto> regionList = childListMap.get(region.getId());
            regionResDto.setChildren(regionList);
            result.add(regionResDto);
            //存入緩存
            JedisUtils.lpush(key, JsonUtils.objectToJson(regionResDto));
        }