Problem:
Score : 300 points

Problem Statement

You are given an undirected unweighted graph with N vertices and M edges that contains neither self-loops nor double edges.

Here, a self-loop is an edge where a_i = b_i (1≤i≤M), and double edges are two edges where (a_i,b_i)=(a_j,b_j) or (a_i,b_i)=(b_j,a_j) (1≤i<j≤M).

How many different paths start from vertex 1 and visit all the vertices exactly once?

Here, the endpoints of a path are considered visited.

For example, let us assume that the following undirected graph shown in Figure 1 is given.

Figure 1: an example of an undirected graph

The following path shown in Figure 2 satisfies the condition.

Figure 2: an example of a path that satisfies the condition

However, the following path shown in Figure 3 does not satisfy the condition, because it does not visit all the vertices.

Figure 3: an example of a path that does not satisfy the condition

Neither the following path shown in Figure 4, because it does not start from vertex 1.

Figure 4: another example of a path that does not satisfy the condition

Constraints

2≦N≦8

0≦M≦N(N-1)/2

1≦a_i<b_i≦N

The given graph contains neither self-loops nor double edges.

Input

The input is given from Standard Input in the following format:

N M
a_1 b_1
a_2 b_2
:
a_M b_M

Output

Print the number of the different paths that start from vertex 1 and visit all the vertices exactly once.

Sample Input 1

3 3
1 2
1 3
2 3

Sample Output 1

2

The given graph is shown in the following figure:

The following two paths satisfy the condition:

Sample Input 2

7 7
1 3
2 7
3 4
4 5
4 6
5 6
6 7

Sample Output 2

1

This test case is the same as the one described in the problem statement.

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

import Control.Applicative
import Control.Monad
import Data.List
import Data.Array.IArray
import Data.Graph (buildG)
import qualified Data.Sequence as S

bfs :: Int -> Array Int [Int] -> [[Int]]
bfs n g = walk queue
  where
    queue = S.fromList [[1]]
    walk s
        | S.null s = []
        | (xs S.:< s') <- S.viewl s
            = if length xs == n
                then xs : walk s'
                else walk (s' S.>< S.fromList (f xs))
      where
        f ys = zipWith (:) (filter (`notElem` ys) $ g ! head ys) (repeat ys)

main :: IO ()
main = do
    [n, m] <- fmap read . words <$> getLine
    as <- fmap concat . replicateM m $ do
        [a, b] <- fmap read . words <$> getLine
        return [(a, b), (b, a)]
    let g = buildG (1, n) as
    print $ length $ bfs n g