flatten implementation for ruby

array.rb

input = [[1, 2], 3, 4, 5, [6, 7, [8, [10, 11],9]]]
expected = input.dup.flatten

class Array
  def flatten2!
    self.replace(flatten2)
  end
  
  def flatten2(array = self, result = [])
    array.each { |a| a.is_a?(Array) ? flatten2(a, result) : result << a }
    
    result  
  end
end

input.flatten2 == expected.flatten