def merge_sort(elements)
  if elements.size <= 1
    return elements
  else
    left = elements[0,(elements.size / 2)]
    right = elements[(elements.size / 2),elements.size]
    sort(merge_sort(left), merge_sort(right))
  end
end

def sort(arr1, arr2)
  if arr1.empty?
    arr2
  elsif arr2.empty?
    arr1
  elsif arr1[0] <= arr2[0]
    Array(arr1[0]) + sort(arr1[1..-1], arr2)
  else
    Array(arr2[0]) + sort(arr1, arr2[1..-1])
  end
end

elements_to_sort = [10,1,5,3,9,4,2]
p merge_sort(elements_to_sort) == elements_to_sort.sort