Top K Frequent Elements

Given an integer array `nums` and an integer `k`, return _the_ `k` _most frequent elements_. You may return the answer in **any order**.   **Example 1:** **Input:** nums = [1,1,1,2,2,3], k = 2 **Output:** [1,2] ``` **Example 2:** **Input:** nums = [1], k = 1 **Output:** [1] ```   **Constraints:** - `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**.   **Follow up:** Your algorithm's time complexity must be better than `O(n log n)`, where n is the array's size.