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Given a non-empty array of integers, return the k most frequent elements.
Input: nums = [1,1,1,2,2,3], k = 2 Output: [1,2]
Input: nums = , k = 1 Output: 
- 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.
- It's guaranteed that the answer is unique, in other words the set of the top k frequent elements is unique.
- You can return the answer in any order.