public List<Integer> topKFrequent(int[] nums, int k) {

         HashMap<Integer,Integer> map = new HashMap<>();

         for(int num : nums){

             map.put(num, map.getOrDefault(num, 0) + 1);

         }

         //桶排序

         List<Integer>[] buckets = new ArrayList[nums.length + 1];

         for(int key : map.keySet()){

             int frequency = map.get(key);   //元素出现的频率

             if(buckets[frequency] == null){

                 buckets[frequency] = new ArrayList<>();

             }

             buckets[frequency].add(key);    //出现频率相同的元素存入同一个桶中

         }

         //倒序遍历数组，取倒数的前 k 个元素

         List<Integer> ans = new ArrayList<>();

         for(int i = buckets.length - 1; i >= 0; i--){

             if(k <= 0) break;

             if(buckets[i] == null) continue;

             for(int num : buckets[i]){

                 if(k <= 0) break;

                 ans.add(num);

                 k--;

             }

         }

         return ans;

     }