LeetCode 347: Top K Frequent Elements

Date: January 19, 2026
Category: Array | HashMap | Bucket Sort
Time Taken: 45 minutes
Difficulty: Medium

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Problem Statement

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

Link: Top K Frequent Elements

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My Approach

Initial thought: I was unable to think up of any solution.

Final solution:

• First, create a HashMap counter that will count the occurrences of all the numbers in the nums list and use a for loop to count them or can use Collections.Counter.

• After this, create freq a list of lists that will store all the numbers that appeared times equal to index as a list. Fix the length of freq to be equal to len(nums) + 1 .

• Create a list resultList that will store all the top k elements, we will start the for loop at the end of freq until we reach 0 index. Inside this make another for loop that will loop in the internal list of freq and append the numbers in to the resultList.

This is called Bucket Sort solution.

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Solution Code (Bucket Sort)

class Solution:
    def topKFrequent(self, nums: List[int], k: int) -> List[int]:
        # Hashmap to count the occurences of each element
        counter = {}
        for num in nums:
            counter[num] = counter.get(num, 0) + 1

        # List of List to store the elements that occur a particular time, of total length = len(nums) + 1
        freq = [[] for i in range(len(nums) + 1)]

        for number, frequency in counter.items():
            freq[frequency].append(number)

        # Result list that will have the top k elements
        resultList = []
        for i in range(len(freq) - 1, 0, -1):
            for num in freq[i]:
                resultList.append(num)
                if len(resultList) == k:
                    return resultList

Complexity:

Time: O(N + N + N) = O(N)

Space: O(N)

Key Takeaway: Bucket Sorting algorithm.

Pattern: Top K Elements

Mistakes I Made: Did not even figure out a brute force solution.

Series: 90 Days of Data Engineering Progress: 10/90 problems completed

Tags: #DEQuest #LeetCode #Python #DataEngineering #BuildInPublic