Introduction

The problem asks about a frequency of array elements. This means we should start from getting the frequency. Then, sort the elements based on the frequencies.

Problem Description

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

Constraints:

• 1 <= nums.length <= 10**5
• -10**4 <= nums[i] <= 10**4
• k is in the range [1, the number of unique elements in the array].
• It is guaranteed that the answer is unique.

https://leetcode.com/problems/top-k-frequent-elements/

Examples

Example 1
Input: nums = [1,1,1,2,2,3], k = 2
Output: [1,2]

Example 2
Input: nums = [1], k = 1
Output: [1]

Analysis

The first step is to get a frequency list. The following steps are: sort by frequency, then get first k elements. The first solution uses heap’s push and pop, which is a basic approach. The second solution uses Python heap’s nlargest function. It is straightforward and effective, but only when the function came up in our mind.

Solution

class TopKFrequentElements:
    def topKFrequent(self, nums: List[int], k: int) -> List[int]:
        counter = collections.Counter(nums)
        queue = []
        for num, counts in counter.items():
            heapq.heappush(queue, (-counts, num))
        result = []
        for _ in range(k):
            _, num = heapq.heappop(queue)
            result.append(num)
        return result

class TopKFrequentElements:
    def topKFrequent(self, nums: List[int], k: int) -> List[int]:
        counter = collections.Counter(nums)
        return heapq.nlargest(k, counter.keys(), key=counter.get)

Complexities

• Time: O(nlog(n))
• Space: O(n)