Top K Frequent Elements

The problem

Given an integer array nums and an integer k, return the k most frequent elements.

Function: def topKFrequent(nums,k):

Example input (a JSON list of arguments): [[1, 1, 1, 2, 2, 3], 2]
Expected output: [1, 2]

Inputs satisfy the problem statement. Sequences contain at most 200 elements unless a smaller bound is stated. Use JSON values and the supplied function signature.

Hints

1. Count each distinct value, rank by descending frequency, and break equal-frequency ties by ascending numeric value for a deterministic selected set.
2. Check the supplied examples, empty/boundary cases and the stated answer policy before optimizing.

Worked answer

Try it yourself first. When you are ready, open the answer below.

def topKFrequent(nums,k):
    counts=Counter(nums)
    return sorted(counts,key=lambda n:(-counts[n],n))[:k]

Why it works

Count each distinct value, rank by descending frequency, and break equal-frequency ties by ascending numeric value for a deterministic selected set. The selected values may be returned in any order.

Complexity: O(n + u log u) time / O(u) space.

Time: O(n + u log u) time · Space: O(u) space