class Triangle
  def initialize(num_rows)
    @num_rows = num_rows
  end

  def rows
    @triangle = [[1]]
    (2..@num_rows).each { |n| @triangle << make_row(n) }
    @triangle
  end

  def make_row(n)
    previous_row = @triangle[n - 2]
    new_row = []
    n.times do |num|
      if num == 0 || num == n - 1
        new_row << 1
      else
        new_row << (previous_row[num - 1] + previous_row[num])
      end
    end
    new_row
  end

  def last
    @triangle[-1]
  end
end