Question

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 in triangle.txt (right click and ‘Save Link/Target As…’), a 15K text file containing a triangle with one-hundred rows.

NOTE: This is a much more difficult version of Problem 18. It is not possible to try every route to solve this problem, as there are 2⁹⁹ altogether! If you could check one trillion ($10^{12}$) routes every second it would take over twenty billion years to check them all. There is an efficient algorithm to solve it. ;o)

Haskell

parse :: String -> [[Integer]]
parse = map (map read . words) . lines

best :: [Integer] -> [Integer]
best row = map maximum choices where
    choices = zipWith (\a b -> a : [b]) row (tail row)

maxStep :: [Integer] -> [Integer] -> [Integer]
maxStep current next = zipWith (+) next (best current)

maxPath :: [[Integer]] -> Integer
maxPath [[x]] = x
maxPath (current:next:rest) = maxPath $ (maxStep current next) : rest

main :: IO ()
main = do
        str <- readFile "/home/zach/code/euler/067/triangle.txt"
        print $ maxPath $ reverse $ parse str

$ ghc -O2 -o maximum-path-sum maximum-path-sum.hs
$ time ./maximum-path-sum
real   0m0.012s
user   0m0.006s
sys    0m0.006s

Python

#!/usr/bin/env python
import os

def find_sum(triangle):
    def get_options(row, index):
        return triangle[row+1][index], triangle[row+1][index+1]
    row = len(triangle) - 2
    while True:
        try:
            for index, node in enumerate(triangle[row]):
                best = max([node + option for option in get_options(row, index)])
                triangle[row][index] = best
            row -= 1
        except:
            return triangle[0][0]

def main():
    triangle = [[int(digit) for digit in line.split()] for line in open(os.path.join(os.path.dirname(__file__), 'triangle.txt')).readlines()]
    print(find_sum(triangle))

if __name__ == "__main__":
    main()

$ time python3 triangle-max.py
real   0m0.023s
user   0m0.023s
sys    0m0.000s