Определение минимальной длины пути в дереве

data Tree a = Nil | Node a (Tree a) (Tree a) deriving (Show)
 
-- Извлекаем из дерева список листьев с высотами
 
getLs :: Tree a -> Int -> [(Int, Tree a)]
getLs Nil n = []
getLs (Node x Nil Nil) n = [(n, (Node x Nil Nil))]
getLs (Node x r l) n     = (getLs l (n+1)) ++ (getLs r (n+1)) 
 
-- Ищем минимум
 
task :: Tree a -> Tree a
task tree = snd (foldl (\ acc z -> if (fst z) < (fst acc) then z else acc) (head ls) (tail ls))
            where ls=getLs tree 0
 
 
Main> task (Node 1 (Node 0  Nil Nil) (Node 5 (Node 3 Nil Nil) (Node 6 Nil Nil)))
 
Node 0 Nil Nil     -- верно

data Tree a = Nil | Node a (Tree a) (Tree a) deriving (Show)
 
getLs :: Tree a -> Int -> [(Int, Tree a)]
getLs Nil n = []
getLs (Node x Nil Nil) n = [(n, (Node x Nil Nil))]
getLs (Node x r l) n     = (getLs l (n+1)) ++ (getLs r (n+1)) 
 
task :: Tree a -> (Int, Tree a)
task tree = foldl (\ acc z -> if (fst z) < (fst acc) then z else acc) (head ls) (tail ls)
            where ls=getLs tree 0
 
Main> task (Node 1 (Node 0  Nil Nil) (Node 5 (Node 3 Nil Nil) (Node 6 Nil Nil)))
(1,Node 0 Nil Nil)

task tree = foldl1 (\ zMin@(nMin,leafMin) z@(n,leaf) -> if n < nMin then z else zMin) ls
    where ls = getLs tree

data Tree a = Node a [Tree a]
 
find tree = loop [(tree,0)]
    where
        loop [] = error "O_o"
        loop ((Node a [],depth):t) = (a,depth)
        loop ((Node _ childs,depth):t) = loop (t ++ map (\c -> (c,depth+1)) childs)
 
main = do
    let tree = Node 0 [Node 1 [Node 4 [Node 6 []], Node 5 [Node 7 []]], Node 2 [Node 8 [], Node 9 [Node 10 []]]]
    print$ find tree