Published on: 04/25/2024

Solution:

The idea is simple. Build an array lof 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;
}