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]
Note:
<li>You may assume <i>k</i> is always valid, 1 ≤ <i>k</i> ≤ number of unique elements.</li>
<li>Your algorithm's time complexity <b>must be</b> better than O(<i>n</i> log <i>n</i>), where <i>n</i> is the array's size.</li>