プログラマ脳を鍛える数学パズル 問題14

longest-shiritori-pattern.hs

import           Data.List

main :: IO ()
main = print $ solve [ "def", "fdc", "abcd", "char", "hoge"]

solve :: [String] -> [String]
solve wrds = snd $ last $ sortOn fst $ map (\ws -> (calc ws, ws)) $ permutations wrds

calc :: [String] -> Int
calc [] = 0
calc wrds = let wrd = head wrds
            in _calc (tail wrds) (last wrd) 1

_calc :: [String] -> Char -> Int -> Int
_calc [] _ acc = acc
_calc wrds c acc = let wrd = head wrds
                       h = head wrd
                       l = last wrd
                   in if h == c
                      then _calc (tail wrds) l (acc+1)
                      else _calc [] l acc

output: ["abcd","def","fdc","char","hoge"]