Top K Frequent Elements

 

import java.util.*;

public class Solution {

    public List<Integer> topKFrequent(int[] nums, int k) {

        HashMap<Integer, Integer> numberOccur = getNumOccur(nums);

        PriorityQueue<Integer> priorityQueue = new PriorityQueue<>((o1, o2) -> numberOccur.get(o1) - numberOccur.get(o2));

        Set<Map.Entry<Integer, Integer>> entries = numberOccur.entrySet();
        for(Map.Entry<Integer, Integer> entry:entries){
            priorityQueue.add(entry.getKey());
            if(priorityQueue.size()>k){
                priorityQueue.poll();
            }
        }
        ArrayList<Integer> result = new ArrayList<>();

        for (int i = 0; i < k; i++) {
            result.add(priorityQueue.poll());
        }
        Collections.reverse(result);
        return result;
    }

    private HashMap<Integer, Integer> getNumOccur(int[] nums) {
        HashMap<Integer, Integer> numberOccur = new HashMap<>();
        for (int num : nums) {
            if (numberOccur.get(num) == null) {
                numberOccur.put(num, 0);
            }
            numberOccur.put(num, numberOccur.get(num) + 1);
        }
        return numberOccur;
    }

}