Problem:
Score : 100 points

Problem Statement

There is a directed graph G with N vertices and M edges.
The vertices are numbered 1, 2, \ldots, N, and for each i (1 \leq i \leq M), the i-th directed edge goes from Vertex x_i to y_i.
G does not contain directed cycles.

Find the length of the longest directed path in G.
Here, the length of a directed path is the number of edges in it.

Constraints

All values in input are integers.

2 \leq N \leq 10^5

1 \leq M \leq 10^5

1 \leq x_i, y_i \leq N

All pairs (x_i, y_i) are distinct.

G does not contain directed cycles.

Input

Input is given from Standard Input in the following format:

N M
x_1 y_1
x_2 y_2
:
x_M y_M

Output

Print the length of the longest directed path in G.

Sample Input 1

4 5
1 2
1 3
3 2
2 4
3 4

Sample Output 1

3

The red directed path in the following figure is the longest:

Sample Input 2

6 3
2 3
4 5
5 6

Sample Output 2

2

The red directed path in the following figure is the longest:

Sample Input 3

5 8
5 3
2 3
2 4
5 2
5 1
1 4
4 3
1 3

Sample Output 3

3

The red directed path in the following figure is one of the longest:

Solution in Haskell (GHC 7.10.3) (judge verdict: Accepted)

{-# OPTIONS_GHC -O2 #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE BangPatterns #-}
{-# LANGUAGE DeriveFunctor #-}
{-# LANGUAGE TypeSynonymInstances #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FlexibleContexts #-}
module Main where

import Control.Arrow ((***), (&&&), first, second)
import Control.Monad (replicateM, forM_)
import Data.Array ((!))
import Data.Bool (bool)
import Data.Char (isSpace)
import Data.Function (on)
import Data.Graph (Vertex, Edge, buildG, topSort)
import Data.List (unfoldr, foldl', sort, (\\), delete)
import qualified Data.Map as Map
import qualified Data.Set as Set
import qualified Data.Vector.Unboxed as U
import qualified Data.Vector.Unboxed.Mutable as UM
import qualified Data.Vector as V
import qualified Data.Vector.Mutable as VM
import qualified Data.ByteString.Char8 as C
import qualified Data.ByteString.Unsafe as B
import System.IO (hPutStr, hPutStrLn, stdin, stdout, withFile, IOMode(..))

type Parser a = C.ByteString -> Maybe (a, C.ByteString)

parseInt :: Parser Int
parseInt = C.readInt . C.dropWhile isSpace

getInts :: IO [Int]
getInts = unfoldr parseInt <$> C.getLine

getIntVec :: Int -> IO (U.Vector Int)
getIntVec n = U.unfoldrN n parseInt <$> C.getLine

---------------------------------------------------------
swap (x, y) = (y, x)
pair (f, g) x = (f x, g x)
cross (f, g) (x, y) = (f x, g y)

newtype Fix f = In { out :: f (Fix f) }

data Hisx f a x = Hisx { unHisx :: (a, f x) } deriving (Show, Functor)
newtype Cofree f a = Cf { unCf :: Fix (Hisx f a) }
instance Functor f => Functor (Cofree f) where
  fmap f = Cf . ana (phi . out) . unCf
    where phi (Hisx (a, x)) = Hisx (f a, x)

extract :: Functor f => Cofree f t -> t
extract cf = case out (unCf cf) of
  Hisx (a, _) -> a

sub :: Functor f => Cofree f a -> f (Cofree f a)
sub cf = case out (unCf cf) of
  Hisx (_, b) -> fmap Cf b

data Futx f a x = Futx { unFutx :: Either a (f x) } deriving (Show, Functor)
newtype Free f a = Fr { unFr :: Fix (Futx f a) }
instance Functor f => Functor (Free f) where
  fmap f = Fr . cata (In . phi) . unFr
    where phi (Futx (Left a)) = Futx (Left (f a))
          phi (Futx (Right x)) = Futx (Right x)

inject :: a -> Free f a
inject = Fr . In . Futx . Left

-- catamorphism
cata :: Functor f => (f a -> a) -> Fix f -> a
cata phi = phi . fmap (cata phi) . out
-- anamorphism
ana :: Functor f => (a -> f a) -> a -> Fix f
ana psi = In . fmap (ana psi) . psi
-- hylomorphism
hylo :: Functor f => (f b -> b) -> (a -> f a) -> a -> b
hylo phi psi = phi . fmap (hylo phi psi) . psi -- cata phi . ana psi
-- metamorphism
meta :: Functor f => (f a -> a) -> (a -> f a) -> Fix f -> Fix f
meta phi psi = In . fmap (meta phi psi) . out -- ana psi . cata phi
-- paramorphism
para :: Functor f => (f (Fix f, t) -> t) -> Fix f -> t
para phi = phi . fmap (pair (id, para phi)) . out
-- apomorphism
apo :: Functor f => (t -> f (Either (Fix f) t)) -> t -> Fix f
apo psi = In . fmap (uncurry either (id, apo psi)) . psi
-- histomorphism
histo :: Functor f => (f (Cofree f t) -> t) -> Fix f -> t
histo phi = extract . cata ap
  where ap = cast . Hisx . pair (phi, id)
        cast = Cf . In . fmap unCf
-- futumorphism
futu :: Functor f => (t -> f (Free f t)) -> t -> Fix f
futu psi = ana ap . inject
  where ap = uncurry either (psi, id) . unFutx . cast
        cast = fmap Fr . out . unFr
-- chronomorphism
chrono :: Functor f => (f (Cofree f b) -> b) -> (a -> f (Free f a)) -> a -> b
chrono phi psi = extract . hylo phi' psi' . inject
  where
    phi' = toCofree . Hisx . pair (phi, id)
    toCofree = Cf . In . fmap unCf
    psi' = uncurry either (psi, id) . unFutx . fromFree
    fromFree = fmap Fr . out . unFr
-- cochronomorphism
cochrono :: Functor f => (f (Cofree f t) -> t) -> (t -> f (Free f t)) -> Fix f -> Fix f
cochrono phi psi = futu psi . histo phi
-- zygomorphism
zygo :: Functor f => (f a -> a) -> (f (a, b) -> b) -> Fix f -> b
zygo f phi = snd . cata (pair (f . fmap fst, phi))
-- cozygomorphism
cozygo :: Functor f => (a -> f a) -> (b -> f (Either a b)) -> b -> Fix f
cozygo f psi = ana (uncurry either (fmap Left . f, psi)) . Right
-- dynamorphism
dyna :: Functor f => (f (Cofree f b) -> b) -> (a -> f a) -> a -> b
dyna f g = chrono f (fmap inject . g) -- histo f . ana g
-- codynamorphism
codyna :: Functor f => (f b -> b) -> (a -> f (Free f a)) -> a -> b
codyna f g = chrono (f . fmap extract) g
-- mutumorphism
mutu :: Functor f => (a -> b) -> (f a -> a) -> Fix f -> b
mutu proj phi = proj . cata phi
-- comutumorphism
comutu :: Functor f => (b -> a) -> (a -> f a) -> b -> Fix f
comutu proj psi = ana psi . proj


foldn (c, f) 0 = c
foldn (c, f) n = f (foldn (c, f) (n-1))

paran (c, f) 0 = c
paran (c, f) n = f n (paran (c, f) (n-1))

---------------------------------------------------------

getTuple :: IO Edge
getTuple = do
  (x:y:_) <- getInts
  return (x, y)

getIdxTuple :: IO Edge
getIdxTuple = cross (pred, pred) <$> getTuple

getProblem :: IO (Int, Int, U.Vector Edge)
getProblem = do
  (n, m) <- getTuple
  xys <- U.replicateM m getTuple
  return (n, m, xys)

data TreeF a x = Tip a | Node a [x] deriving (Show, Functor)
type Tree a = Fix (TreeF a)
instance Show a => Show (Tree a) where
  show (In (Tip x)) = "Tip " ++ show x
  show (In (Node a xs)) = "Node " ++ show a ++ " " ++ show xs

tip :: a -> Tree a
tip a = In (Tip a)
node :: a -> [Tree a] -> Tree a
node a xs = In (Node a xs)

tip' :: b -> a -> Cofree (TreeF a) b
tip' c n = Cf (In (Hisx (c, Tip n)))
node' :: ([a] -> a) -> b -> [Cofree (TreeF b) a] -> Cofree (TreeF b) a
node' f a xs = Cf (In (Hisx (f (map extract xs), Node a (map unCf xs))))

main :: IO ()
main = do
  (n, m, xys) <- getProblem
  print . fst . extract $ solve n xys


solve n xys = node' (second reverse.maximum) 0 (Map.elems m)
  where
    mkT :: Vertex -> Cofree (TreeF Vertex) (Int, [Vertex])
    mkT i = tip' (0,[i]) i
    mkN :: Vertex -> [Cofree (TreeF Vertex) (Int, [Vertex])] -> Cofree (TreeF Vertex) (Int, [Vertex])
    mkN i = node' (((+1) *** (i:)).maximum) i

    g = buildG (1,n) $ U.toList xys
    sorted = topSort g
    entry i m =
      let xs = g ! i
      in if null xs
         then Map.insert i (mkT i) m
         else Map.insert i (mkN i (map (m Map.!) xs)) m
    m = foldr entry Map.empty sorted

ex1 :: (Int, [Int])
ex1 = extract top
  where
    top = node' (second reverse.maximum) 0 [nd1,nd2,nd3,nd4]
    mkT :: Vertex -> Cofree (TreeF Vertex) (Int, [Vertex])
    mkT i = tip' (0,[i]) i
    mkN :: Vertex -> [Cofree (TreeF Vertex) (Int, [Vertex])] -> Cofree (TreeF Vertex) (Int, [Vertex])
    mkN i = node' (((+1) *** (i:)).maximum) i
    nd1 = mkN 1 [nd3,nd2]
    nd3 = mkN 3 [nd4,nd2]
    nd2 = mkN 2 [nd4]
    nd4 = mkT 4

ex2 :: (Int, [Int])
ex2 = extract top
  where
    top = node' (second reverse.maximum) 0 [nd1,nd2,nd3,nd4,nd5,nd6]
    mkT :: Vertex -> Cofree (TreeF Vertex) (Int, [Vertex])
    mkT i = tip' (0,[i]) i
    mkN :: Vertex -> [Cofree (TreeF Vertex) (Int, [Vertex])] -> Cofree (TreeF Vertex) (Int, [Vertex])
    mkN i = node' (((+1) *** (i:)).maximum) i
    nd4 = mkN 4 [nd5]
    nd5 = mkN 5 [nd6]
    nd6 = mkT 6
    nd2 = mkN 2 [nd3]
    nd3 = mkT 3
    nd1 = mkT 1

ex3 :: (Int, [Int])
ex3 = extract top
  where
    top = node' (second reverse.maximum) 0 [nd1,nd2,nd3,nd4,nd5]
    mkT :: Vertex -> Cofree (TreeF Vertex) (Int, [Vertex])
    mkT i = tip' (0,[i]) i
    mkN :: Vertex -> [Cofree (TreeF Vertex) (Int, [Vertex])] -> Cofree (TreeF Vertex) (Int, [Vertex])
    mkN i = node' (((+1) *** (i:)).maximum) i
    nd5 = mkN 5 [nd1,nd2,nd3]
    nd2 = mkN 2 [nd4,nd3]
    nd1 = mkN 1 [nd3,nd4]
    nd4 = mkN 4 [nd3]
    nd3 = mkT 3