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19. Top K Frequent Elements

mediumAsked at Instacart

Return the k most frequently occurring integers — Instacart uses frequency ranking everywhere from surfacing top-ordered items to ranking stores by pickup volume in real-time catalog queries.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

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, number of unique elements in the array]
  • The answer is guaranteed to be unique

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]

Approaches

1. Sort by frequency

Count frequencies with a hash map, then sort the unique elements by their count descending and return the first k.

Time
O(n log n)
Space
O(n)
function topKFrequent(nums, k) {
  const freq = new Map();
  for (const n of nums) freq.set(n, (freq.get(n) || 0) + 1);
  return [...freq.keys()].sort((a, b) => freq.get(b) - freq.get(a)).slice(0, k);
}

Tradeoff:

2. Bucket sort (linear)

Map each element to its frequency, then place elements into buckets indexed by frequency. Collect from the highest-frequency bucket down until k elements are gathered.

Time
O(n)
Space
O(n)
function topKFrequent(nums, k) {
  const freq = new Map();
  for (const n of nums) freq.set(n, (freq.get(n) || 0) + 1);

  // buckets[i] = list of elements that appear exactly i times
  const buckets = Array.from({ length: nums.length + 1 }, () => []);
  for (const [num, count] of freq) buckets[count].push(num);

  const result = [];
  for (let i = buckets.length - 1; i >= 1 && result.length < k; i--) {
    result.push(...buckets[i]);
  }
  return result.slice(0, k);
}

Tradeoff:

Instacart-specific tips

Instacart's catalog and recommendation pipelines rank items by order frequency under tight latency budgets. The bucket-sort approach signals you can reason about frequency bounds (max freq = n) to shave a log factor — interviewers respond well when you call that out before coding.

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Output

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