https://leetcode.com/problems/top-k-frequent-elements/description/
Given an integer array nums and an integer k, return the k most frequent elements. You may return the answer in any order.
Input: nums = [1,1,1,2,2,3], k = 2
Output: [1,2]
Input: nums = [1], k = 1
Output: [1]
1 <= nums.length <= 105
-104 <= nums[i] <= 104
k is in the range [1, the number of unique elements in the array].
It is guaranteed that the answer is unique.
Your algorithm's time complexity must be better than O(n log n), where n is the array's size.