Snippet to convert Array to Hash with duplicate elements combined together

array_to_hash.rb

def array_to_hash(arr)
  hash_val = Hash.new
  arr.uniq.each {|n| hash_val[n] = arr.grep(n)}
  hash_val
end

# array_to_hash([1,2,3,2,1]) => {1=>[1, 1], 2=>[2, 2], 3=>[3]}