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19. Top K Frequent Elements

mediumAsked at Riot Games

Return the k most frequent elements in an array — a Riot favorite for telemetry-rollup and anti-cheat anomaly detection pipelines.

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 and may assume the answer is unique.

Constraints

  • 1 <= nums.length <= 10^5
  • k is in the range [1, number of unique 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]

Approaches

1. Hash + full sort

Count frequencies then sort all unique keys by count descending.

Time
O(n log n)
Space
O(n)
const c = new Map();
for (const n of nums) c.set(n, (c.get(n)||0)+1);
return [...c.entries()].sort((a,b)=>b[1]-a[1]).slice(0,k).map(e=>e[0]);

Tradeoff:

2. Bucket sort by frequency

Use n+1 buckets indexed by frequency, then walk buckets from high to low collecting k items. Riot uses the same bucket trick to aggregate per-tick cheat-signal counts without log-factor sorts.

Time
O(n)
Space
O(n)
function topKFrequent(nums, k) {
  const c = new Map();
  for (const n of nums) c.set(n, (c.get(n)||0)+1);
  const buckets = Array.from({length:nums.length+1},()=>[]);
  for (const [v,f] of c) buckets[f].push(v);
  const out = [];
  for (let i=buckets.length-1; i>=0 && out.length<k; i--) {
    for (const v of buckets[i]) { out.push(v); if (out.length===k) break; }
  }
  return out;
}

Tradeoff:

Riot Games-specific tips

Riot rewards candidates who recognize bucket sort's O(n) win when counts are bounded — the same constraint their anti-cheat heuristic counters operate under per server tick.

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Output

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