new_contents: // 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].
  // Given [1,1,1,2,2,3] and k = 1, return [1].
  
  // 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.
  
subject: add desc