18.rb

arr = "75
      95 64
      17 47 82
      18 35 87 10
      20 04 82 47 65
      19 01 23 75 03 34
      88 02 77 73 07 63 67
      99 65 04 28 06 16 70 92
      41 41 26 56 83 40 80 70 33
      41 48 72 33 47 32 37 16 94 29
      53 71 44 65 25 43 91 52 97 51 14
      70 11 33 28 77 73 17 78 39 68 17 57
      91 71 52 38 17 14 91 43 58 50 27 29 48
      63 66 04 68 89 53 67 30 73 16 69 87 40 31
      04 62 98 27 23 09 70 98 73 93 38 53 60 04 23"

a = arr.split("\n").map(&:strip).map{|x| x.split(' ')}.map{ |row| row.map{ |cell| cell.to_i } }

def max(first, second)
  first > second ? first : second
end

order = (-a.size..-2).to_a.reverse
order.each do |row|
  a[row].map!.with_index do |cell, index|
    cell + max(a[row+1][index], a[row+1][index+1])
  end
end

puts a[0][0]