Given an array of integers, arr, and an integer, k, return the $k$ most frequent elements.

Note: You can return the answer in any order.

Constraints:

• $1$ $\leq$ arr.length $\leq$ $10^{3}$
• $10^{-4}$ $\leq$ arr[i] $\leq$ $10^{4}$
• 1 $\leq$ k $\leq$ number of unique elements in an array.
• It is guaranteed that the answer is unique.

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