LeetCode-in-All

347. Top K Frequent Elements

Medium

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

Example 1:

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

Output: [1,2]

Example 2:

Input: nums = [1], k = 1

Output: [1]

Constraints:

• 1 <= nums.length <= 105
• -104 <= nums[i] <= 104
• k is in the range [1, the number of unique elements in the array].
• It is guaranteed that the answer is unique.

Follow up: Your algorithm’s time complexity must be better than O(n log n), where n is the array’s size.

Solution

(define (hash-table-counter ls)
  (let ([counts (make-hash)])
    (for-each (λ (v) (hash-update! counts v add1 0)) ls)
    counts))

(define (top-k-frequent nums k)
  (let ([counter (hash-table-counter nums)])
    (map car (take (sort (hash->list counter) #:key cdr >) k))))

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