Idea is simple. Build a array of list to be buckets with length 1 to sort.

```
public List<Integer> topKFrequent(int[] nums, int k) {
List<Integer>[] bucket = new List[nums.length + 1];
Map<Integer, Integer> frequencyMap = new HashMap<Integer, Integer>();
for (int n : nums) {
frequencyMap.put(n, frequencyMap.getOrDefault(n, 0) + 1);
}
for (int key : frequencyMap.keySet()) {
int frequency = frequencyMap.get(key);
if (bucket[frequency] == null) {
bucket[frequency] = new ArrayList<>();
}
bucket[frequency].add(key);
}
List<Integer> res = new ArrayList<>();
for (int pos = bucket.length - 1; pos >= 0 && res.size() < k; pos--) {
if (bucket[pos] != null) {
res.addAll(bucket[pos]);
}
}
return res;
}
```