Top K Frequent Elements

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

For example,
Given [1,1,1,2,2,3] and k = 2, return [1,2].

Note: 

• You may assume k is always valid, 1 ≤ k ≤ number of unique elements.
• Your algorithm's time complexity must be better than O(n log n), where n is the array's size.

• public class Solution {
      public List<Integer> topKFrequent(int[] nums, int k) {
          HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
          for (int num : nums) {
              if ( map.containsKey(num) )
                  map.put( num, map.get(num) + 1);
              else 
                  map.put( num, 1 );
          }
          
          PriorityQueue<Map.Entry<Integer, Integer> > pq = new PriorityQueue<>(
              new Comparator<Map.Entry<Integer, Integer> >() {
                  public int compare(Map.Entry<Integer, Integer> a, Map.Entry<Integer, Integer> b) {
                      return b.getValue() - a.getValue();
                  }
              } 
          );
          pq.addAll(map.entrySet());
          
          List<Integer> result = new ArrayList<>();
          for (int i = 0; i < k; i++) {
              if (!pq.isEmpty()) {
                  result.add(pq.poll().getKey());
              } else break;
          }
          return result;
      }
  }