Skip to main content

17. Top K Frequent Elements

mediumAsked at Coinbase

Surface the K most-traded assets on an exchange — Coinbase uses this problem to test whether you reach for a heap over a naive sort when ranking trade volumes across thousands of instruments.

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
  • k is in the range [1, the 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]

Explanation: 1 appears 3 times, 2 appears 2 times — both top-2.

Example 2

Input
nums = [1], k = 1
Output
[1]

Approaches

1. Sort by frequency

Build a frequency map, collect entries, sort descending by count, return first k keys.

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.entries()]
    .sort((a, b) => b[1] - a[1])
    .slice(0, k)
    .map(([num]) => num);
}

Tradeoff:

2. Bucket sort (optimal)

Use an array of buckets indexed by frequency (max = n). Iterate buckets from high to low to collect k results in O(n).

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);

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

Tradeoff:

Coinbase-specific tips

Coinbase interviewers pay close attention to whether you recognise the bucket-sort escape hatch. When the range of possible frequencies is bounded by n, you can beat the O(n log n) sort — a signal that you think about constraints the way trading-system engineers think about throughput ceilings.

Solve it now

Free. No sign-up. Python and JavaScript run instantly in your browser.

Output

Press Run or Cmd+Enter to execute

Practice these live with InterviewChamp.AI

Drill Top K Frequent Elements and other Coinbase interview questions under real-loop conditions with instant feedback on your reasoning, complexity claims, and code.

Practice these live with InterviewChamp.AI →