dfs.hs

import Data.Array.IArray

type G = Array Int [Int]

g1 :: G
g1 = array (1, 4) [[], [1], [1], [2, 3]]

preds :: Int -> G -> [Int]
preds x g = g !! x


dfs :: Int -> G -> Array Int [Int]
dfs o g = r
    where r = array (1, size g) [go x | x <- [1..size g]]
          go x = if x == o then [x]
                           else case preds x g of
                                    []     -> [x]
                                    (s, _) -> (x:r!!s)


main = print $ dfs g1