leetcode347. Top K Frequent Elements

Given a non-empty array of integers, return the k most frequent elements.

For example,
Given [1,1,1,2,2,3] and k = 2, return [1,2].

Note: 
You may assume k is always valid, 1 ≤ k ≤ number of unique elements.
Your algorithm's time complexity must be better than O(n log n), where n is the array's size.

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