Top K Frequent Elements

You are given a list of integers `nums` and an integer `k`. Return the `k` elements that appear most often in the list. Return them in order of decreasing frequency; when several elements have the same frequency, return the smaller elements first. Input: `nums` (a list of integers) and `k` (an integer). Output: a list of the k most frequent elements. Example: nums = [1, 1, 1, 2, 2, 3], k = 2 -> [1, 2], because 1 appears three times and 2 appears twice.

def top_k_frequent(nums: list[int], k: int) -> list[int]:
    """Return the k most frequent elements, most frequent first."""
    pass