347. Top K Frequent Elements

class Solution {
    public int[] topKFrequent(int[] nums, int k) {
        if (k == nums.length) {
            return nums;
        }
        
        // 1. build hash map : character and how often it appears
        // O(N) time
        Map<Integer, Integer> count = new HashMap();
        for (int n: nums) {
          count.put(n, count.getOrDefault(n, 0) + 1);
        }

        // init heap 'the less frequent element first'
        Queue<Integer> heap = new PriorityQueue<>(
            (n1, n2) -> count.get(n1) - count.get(n2));

        // 2. keep k top frequent elements in the heap
        // O(N log k) < O(N log N) time
        for (int n: count.keySet()) {
          heap.add(n);
          if (heap.size() > k) {
              heap.poll();    
          }
        }

        // 3. build an output array
        // O(k log k) time
        int[] top = new int[k];
        for(int i = k - 1; i >= 0; i--) {
            top[i] = heap.poll();
        }
        return top;
    }
}

class Solution {
    public List<Integer> topKFrequent(int[] nums, int k) {
        List<Integer> res = new ArrayList<>();
        if (nums == null || nums.length == 0) {
            return res;
        } 
        Map<Integer, Integer> map = new HashMap<>();
        for (int num : nums) {
            if (!map.containsKey(num)) {
                map.put(num, 1);
            } else {
                map.put(num, map.get(num) + 1);
            }
        }
        
        PriorityQueue<Map.Entry<Integer, Integer>> minHeap = new PriorityQueue<>(k, new Comparator<Map.Entry<Integer, Integer>>() {
            @Override
            public int compare(Map.Entry<Integer, Integer> entry1, Map.Entry<Integer, Integer> entry2) {
                return entry1.getValue() - entry2.getValue();
            }
        });
        
        for (Map.Entry<Integer, Integer> entry : map.entrySet()) {
            minHeap.offer(entry);
            if (minHeap.size() > k) {
                minHeap.poll();
            }
        }
        
        while (!minHeap.isEmpty()) {
            res.add(minHeap.poll().getKey());
        }
        
        Collections.reverse(res);
        return res;
        
    }
}