[LC] 347. Top K Frequent Elements

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

Example 1:

Input: nums = [1,1,1,2,2,3], k = 2
Output: [1,2]

Example 2:

Input: nums = [1], k = 1
Output: [1]

class Solution {
    public List<Integer> topKFrequent(int[] nums, int k) {
        Map<Integer, Integer> map = new HashMap<>();
        for (Integer num: nums) {
          map.put(num, map.getOrDefault(num, 0) + 1);
        }
        // keep a top frequency heap
        PriorityQueue<Map.Entry<Integer, Integer>> pq = new PriorityQueue<>((a, b) -> 
          a.getValue() == b.getValue() ? b.getKey().compareTo(a.getKey()): a.getValue() - b.getValue()
        );
        for (Map.Entry<Integer, Integer> entry: map.entrySet()) {
            pq.offer(entry);
            if (pq.size() > k) {
                pq.poll();
            }
        }
        List<Integer> res = new ArrayList<>();
        // while (!pq.isEmpty()) {
        //     res.add(0, pq.poll().getKey());
        // }
        while (!pq.isEmpty()) {
            res.add(pq.poll().getKey());
        }
        return res;
    }
}