My solution to Project Euler 18.

018-maximum-path-sum-i.hs

problem = [
		[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]
	]

big (a, b)
	| a > b = a
	| otherwise = b

-- Start at the bottom, pick the better one at each level
-- (I thought writing that down would help but it really doesn’t)
routeUp :: [Int] -> [Int] -> [Int]
routeUp up down = map (\(i, u) -> u + big (down !! i, down !! (i + 1))) $ zip [0..(length up - 1)] up

main = print $ head $ foldr1 routeUp problem