city_walking.rb

NUMBER_OF_BLOCKS = 20
GRID_SIZE = NUMBER_OF_BLOCKS + 1

previous_row = Array.new(GRID_SIZE, 1)

(GRID_SIZE-1).times do |_|
  previous_row =
    previous_row.each_with_object([]) do |item,current_row|
      current_row << item + (current_row.last || 0)
    end
end

puts "# of unqiue paths: #{previous_row.last}"