Problem1: Top K Frequently Repeating Elements(https://leetcode.com/problems/top-k-frequent-elements/)
from a array of integers, find the k most frequent elements.
Example 1:
Input: nums = [1,1,1,2,3,3], k = 2
Output: [1,3]
Example 2:
Input: nums = [1], k = 1
Output: [1]
Your algorithm's time complexity must be better than O(n log n), where n is the array's size.