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 Go (1.6) (judge verdict: Accepted)

package main

import "fmt"

func main() {
	var n, m int
	fmt.Scan(&n, &m)
	r := make([][]bool, n)
	for i := 0; i < n; i++ {
		r[i] = make([]bool, n)
	}
	var a, b int
	for i := 0; i < m; i++ {
		fmt.Scan(&a, &b)
		a--
		b--
		r[a][b], r[b][a] = true, true
	}

	res := solve(n, r)
	fmt.Println(res)
}

func solve(n int, r [][]bool) int {
	visited := make([]bool, n)

	var dfs func(a int) int
	dfs = func(a int) int {
		ok := true
		for _, v := range visited {
			if !v {
				ok = false
				break
			}
		}
		if ok {
			return 1
		}

		c := 0
		for b := 0; b < n; b++ {
			if r[a][b] && !visited[b] {
				visited[b] = true
				c += dfs(b)
				visited[b] = false
			}
		}

		return c
	}

	visited[0] = true
	return dfs(0)
}

func contains(s []int, e int) bool {
	for _, v := range s {
		if e == v {
			return true
		}
	}
	return false
}