gistfile1.txt

# ca marche
[1, 2, 1, 2, 3, 3].reduce({}) do |memo, i|
  memo[i] ||= []
  memo[i] << i
  memo
end

# ca marche pas
[1, 2, 1, 2, 3, 3].reduce(Hash.new([])) do |memo, i|
  memo[i] << i
  memo
end

# why?