Top K Frequent Elements

Problem

Given an integer array nums and an integer k, return the k most frequent elements. You may return the answer in any order.

Examples

The brute-force approach

Count every element's frequency, then sort the unique elements by their count descending, and return the first k.

counts = {}
for num in nums:
    counts[num] = counts.get(num, 0) + 1

sorted_keys = sorted(counts, key=lambda x: -counts[x])

return sorted_keys[:k]

Sorting all the unique elements takes O(n log n). But you only need the top k, not a full sorted list. There's a way to find the answer in O(n) by using the frequencies as array indices.

Spotting the pattern

This is a Frequency Map problem. The key question to ask yourself:

If I know how often each element appears, how do I find the top k without sorting everything?

Answering that is where it clicks, and it's exactly what the guided walkthrough below builds with you: the pattern reasoning, a progressive hint ladder that never spoils the answer, a row-by-row dry run, the optimized solution, and an in-browser editor to run your code against real test cases.