lucky_tickets.rb

# frozen_string_literal: true

class LuckyTickets
  def self.call(n)
    n = n/2
    n2 = 10 ** n - 1
    a = Array.new(9 * n + 1) { 0 }
    (0..n2).each do |l|
      a[l.to_s.chars.map(&:to_i).sum] += 1
    end
    puts a.map { |e| e*e }.sum
  end
end

result.txt

[1] pry(main)> LuckyTickets.call(6)
55252