347_Top_K_Frequent_Elements.java

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

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

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

    // build output list
    List<Integer> top_k = new LinkedList();
    while (!heap.isEmpty())
      top_k.add(heap.poll());
    Collections.reverse(top_k);
    return top_k;
  }
}