What does Array#tally do?

tally (Ruby 2.7 and later) counts the elements of an array and returns a Hash from each distinct element to its count, so [4, 7, 4].tally is {4 => 2, 7 => 1}. Before 2.7 the idiom was each_with_object(Hash.new(0)) { |n, h| h[n] += 1 }.

Could I use max_by(k) instead of sorting?

Yes: nums.tally.max_by(k) { |_, c| c }.map(&:first) returns the k pairs with the highest counts, most frequent first. It reads well and selects the top k without you writing the sort; at a few thousand distinct values the two finish within a millisecond of each other.

Why Array.new(n) { [] } and not Array.new(n, [])?

Array.new(n, []) fills every slot with the same array object, so pushing into buckets[3] would show up in every bucket. The block form runs once per slot and gives each bucket its own empty array.

Does return inside each leave the whole method?

Yes. A return inside a block returns from the method the block was written in, not just from the block, so the bucket loop stops as soon as k values are collected. Use break or next when you only want to leave the block.

Why write a heap by hand?

Ruby's core and standard library ship no priority queue, and a problem runner cannot install gems. An Array with sift_up and sift_down is about twenty lines; ordering by heap[i][0] keeps the comparison on counts only.

Top K Frequent Elements in Ruby: Sort, Min-Heap and Bucket Sort Solutions

top_k_frequent.rb

Ruby 4.0

Not run yet

+ 7 hidden

Input

nums = [4, 4, 7, 2, 7, 4, 9, 7, 4]
k = 2

Expected

[4, 7]

4 appears four times and 7 three times; 2 and 9 appear once each, so they are left out.