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import Control.Monad
import Data.List
import qualified Data.Map.Strict as Map
type Inventory = Map.Map Int [Int]
findbridges :: Int -> Inventory -> [[(Int, Int)]]
findbridges start inven = case Map.lookup start inven of
Nothing -> [[]]
Just nexts ->
concatMap (\n -> map ((start, n) :) $ findbridges n (delpair start n $ delpair n start inven)) $
nub nexts
where
delpair :: Int -> Int -> Inventory -> Inventory
delpair a b = Map.update (\l -> let l' = l \\ [b] in if null l' then Nothing else Just l') a .
Map.update (\l -> let l' = l \\ [a] in if null l' then Nothing else Just l') b
strength :: [(Int, Int)] -> Int
strength = sum . map (uncurry (+))
parse :: String -> (Int, Int)
parse str = let Just idx = findIndex (== '/') str in (read $ take idx str, read $ drop (succ idx) str)
main :: IO ()
main = do
input <- liftM (map parse . lines) (readFile "24.in")
let mp = foldl (\m (a,b) -> Map.insertWith (++) b [a] $ Map.insertWith (++) a [b] m) Map.empty input
br = findbridges 0 mp
print $ maximum (map strength br)
let maxlen = maximum (map length br)
print $ maximum $ map strength $ filter ((== maxlen) . length) br
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