Top K Frequent Elements

Write `top_k_frequent(nums, k)` returning the `k` most frequent values, in any order. top_k_frequent([1, 1, 1, 2, 2, 3], 2) -> [1, 2] Sorting by count is O(n log n) and a size-k heap is O(n log k). Both are accepted; neither is the best. A frequency can never exceed the length of the list, so counts can be bucketed into an array indexed by frequency and read from the top — O(n). Reach for that one and say why it is available here.