## Project Euler Problem 67 Solution

## 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 2^{99} 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 0m0.019s
user 0m0.012s
sys 0m0.004s
```

## 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 0m0.026s
user 0m0.024s
sys 0m0.000s
```

Questions? Comments? Send me an email: z@chdenton.com