 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.

