18.rb

# Problem 18
# https://projecteuler.net/problem=18

# By starting at the top of the triangle below and moving to adjacent 
# numbers on the row below, the maximum total from top to bottom is 23.

# 3
# 7 4
# 2 4 6
# 8 5 9 3

# That is, 3 + 7 + 4 + 9 = 23.

# Find the maximum total from top to bottom of the triangle below:

# 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

# NOTE: As there are only 16384 routes, it is possible to solve
# this problem by trying every route. 
# However, Problem 67, is the same challenge with a triangle containing one-hundred rows;
# it cannot be solved by brute force, and requires a clever method! ;o)


piramide =
'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'

piramide = piramide.split("\n").map { |line| line.split(' ').map { |s| s.to_i }}

max = 0
[0, 1].repeated_permutation(piramide.size-1) do |n|
  sum = 0
  sum += piramide[0][0]

  second_index = 0
  (piramide.size-1).times do |i|
    second_index += n[i]
    sum += piramide[i+1][second_index]
  end
  max = sum if max < sum
end

puts max