hash_default_hash

data = [1,2,3,2,2,1,4,5,4,3]

result = data.reduce(Hash.new {|hash, key| hash[key] = []}) do |hash, number|
  hash[number] << number
  hash
end

result = {}
data.each do |number|
  result[number] ||= []
  result[number] << number
end
p result