LC 347 - K most Frequent bucket sort - Java - study code - source: http://www.guoting.org/leetcode/leetcode-347-top-k-frequent-elements/

LC347KMostFrequentBucketSort.java

public class Solution {
    public List<Integer> topKFrequent(int[] nums, int k){
        List<Integer>[] bucket=new List[nums.length+1];
        Map<Integer,Integer> frequencyMap=new HashMap<>();
        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;pos--){
            if(bucket[pos]!=null){
                for(Integer i:bucket[pos]){
                    if(res.size()<k){
                        res.add(i);
                    }else{
                        return res;
                    }
                }
            }
        }
        return res;
    }
}