Hacker Rank

AveragePopulation.sql

SELECT country.continent, floor(avg(city.population))
FROM country country
INNER JOIN city city ON city.countrycode = country.code
GROUP BY country.continent

Pairs.rb

def pairs(a,k)
  pairs_with_tail_recursion( a.sort, k, 0)
end

def pairs_with_tail_recursion(list, diff, acm)
  return acm if list.empty?

  element = list[0]
  sublist = list[1..-1]

  acm += sublist.select { |n| n == element - diff || n == element + diff }.count

  pairs_with_tail_recursion(sublist, diff, acm)
end