0xD34F

arr
  .reduce({}){|acc, n| acc.update(n[:books])}
  .collect{|k, v| { author: k, book: v[:book] }}
  .sort_by{|n| n[:author]}

data = data.reduce({}){|acc, (k, v)| acc.update(v['child'] || {k => v})}

(a + b + c).group_by{|n| n[:year]}.map{|i, n| n.reduce({}, :merge)}

(1..height_result - 1).each do |n|
  space_count = (height.to_i - n).abs
  asterisk_count = height_result - space_count * 2 - 1
  print " " * space_count + "*" * asterisk_count + "\n"
end

a = array1.map.with_index{|n, i| '//' + array2[i] + n}.join(' or ')

a = [
  {:car=>["BMW", "Mersedes", "Audi"]},
  {:car=>["Renault", "Ferrari", "Nissan"]},
  {:car=>["VW"]},
  {:country=>["Russia", "Japan", "USA"]},
  {:country=>["China", "Nigeria", "Egypt"]},
  {:city=>["Moscow", "Tokyo", "Amsterdam"]}
]

b = a.group_by{|n| n.keys[0]}.map{|k, n| {k => n.map{|m| m[k]}}}