[LeetCode] 347 - Top K Frequent Elements

LeetCode Problem Link: https://leetcode.com/problems/top-k-frequent-elements/description/

Solution:
class Solution {
    public List<Integer> topKFrequent(int[] nums, int k) {
        // Count the frequency.
        Map<Integer, Integer> counter = new HashMap<Integer, Integer>();
        for (int num : nums) {
            counter.put(num, counter.getOrDefault(num, 0) + 1);
        }
        
        // Store the frequency into buckets.
        List<Integer>[] buckets = new List[nums.length + 1 - (counter.size() - 1)];
        for (int key : counter.keySet()) {
            int count = counter.get(key);
            if (buckets[count] == null) buckets[count] = new ArrayList<Integer>();
            buckets[count].add(key);
        }
        
        // Get top K buckets.
        List<Integer> results = new ArrayList<Integer>();
        for (int i = buckets.length - 1, topK = 0; topK < k && i >= 0; i--) {
            if (buckets[i] != null) {
                results.addAll(buckets[i]);
                topK += buckets[i].size();
            }
        }
        
        return results;
    }

}

Complexity:
O(n) Time complexity.
O(n) Space complexity.

Result:
Run time 24ms
Beats 87% users