Another implementation for Array.flatten

deflate.rb

def deflate(arr, container = [])
  arr.each do |a|
    push(a, container)
  end

  container
end

def push(a, container)
  if a.is_a? Integer
    container << a
  else
    deflate(a, container)
  end
end

p deflate([1, 2, 3]) # [1, 2, 3]
p deflate([[1], [2, 3, 4], 2, 3]) # [1, 2, 3, 2, 3]
p deflate([1, 2, 3, [4, 5]]) # [1, 2, 3, 4, 5]
p deflate([1, 2, 3, [4, 5, [1]]]) # [1, 2, 3, 4, 5, 1]
p deflate([1, 2, 3, [4, 5, [1]], [9, 10], [11, 12, [13]]]) # [1, 2, 3, 4, 5, 1, 9, 10, 11, 12, 13]