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 C# (Mono 4.6.2.0) (judge verdict: Accepted)

#pragma warning disable

using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Numerics;
using System.Runtime.Serialization.Formatters.Binary;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;

static class MainClass
{
    private static Scanner scanner;
    public static void Main(string[] args)
    {
        scanner = new Scanner();
        Solve();
        // for scanner bug
        scanner.Next();
    }

    private static void Solve()
    {
       
        var n = scanner.NextInt();
        var m = scanner.NextInt();
        var perms = NextPermutation(n - 1).Select(x => { x.Add(0); return x; });
        var graph = new bool[n, n];
        for (int i = 0; i < m; i++)
        {
            var a = scanner.NextInt();
            var b = scanner.NextInt();
            graph[a - 1, b - 1] = true;
            graph[b - 1, a - 1] = true;
        }
        var count = 0;
        foreach (var perm in perms)
        {
            var isOk = true;
            var before = perm[0];
            for (int i = 1; i < perm.Count; i++)
            {
                if (!graph[before, perm[i]])
                {
                    isOk = false;
                    break;
                }
                before = perm[i];
            }
            if (isOk)
            {
                count++;
            }
        }
        Console.WriteLine(count);
    }

    private static List<List<int>> NextPermutation(int number)
    {
        if (number == 1)
        {
            return new List<List<int>>() { new List<int>() { 1 } };
        }
        var beforePerm = NextPermutation(number - 1);
        var nextPerm = new List<List<int>>();
        var len = beforePerm[0].Count;
        foreach (var item in beforePerm)
        {
            for (int i = 0; i < len + 1; i++)
            {
                var ni = item.DeepClone();
                ni.Insert(i, number);
                nextPerm.Add(ni);
            }
        }
        return nextPerm;
    }

    #region ライブラリ

    public static long ToInt64(this string str) => long.Parse(str);
    public static bool IsInt32(this string str)
    {
        var ou = 0;
        return int.TryParse(str, out ou);
    }
    public static int ToInt32(this string str) => str == "" ? 0 : int.Parse(str);
    public static double ToDouble(this string str) => double.Parse(str);
    public static BigInteger ToBigInteger(this string str) => BigInteger.Parse(str);
    public static List<string> SplittedStringToList(this string str) => str.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries).ToList();
    public static List<int> SplittedStringToInt32List(this string str) => str.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries).Select(x => int.Parse(x)).ToList();
    public static List<long> SplittedStringToInt64List(this string str) => str.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries).Select(x => long.Parse(x)).ToList();
    public static List<BigInteger> SplittedStringToBigInteger(this string str) => str.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries).Select(x => BigInteger.Parse(x)).ToList();
    public const int INF = int.MaxValue / 4;
    public const long LONGINF = long.MaxValue / 2;
    public const int Mod1e9 = 1000000007;
    public static List<int> ToCumulativeSumList(this List<int> list)
    {
        var ls = new List<int>();
        ls.Add(list[0]);
        var count = list.Count;
        for (int i = 1; i < count; i++)
        {
            ls.Add(list[i] + ls.Last());
        }
        return ls;
    }

    public static void PrintArray(long[,] array)
    {
        for (int i = 0; i < array.GetLength(0); i++)
        {
            var sb = new StringBuilder();
            for (int j = 0; j < array.GetLength(1); j++)
            {
                sb.Append(array[i, j]).Append(" ");
            }
            Console.WriteLine(sb.ToString().Trim());
        }
    }
    public static int Manhattan(int x1, int x2, int y1, int y2)
    {
        return Math.Abs(x1 - x2) + Math.Abs(y1 - y2);
    }

    public static int Manhattan(KeyValuePair<int, int> keyValuePair1, KeyValuePair<int, int> keyValuePair2)
    {
        return Manhattan(keyValuePair1.Key, keyValuePair2.Key, keyValuePair1.Value, keyValuePair2.Value);
    }

    public static T DeepClone<T>(this T src)
    {
        using (var memoryStream = new MemoryStream())
        {
            var binaryFormatter
              = new BinaryFormatter();
            binaryFormatter.Serialize(memoryStream, src);
            memoryStream.Seek(0, SeekOrigin.Begin);
            return (T)binaryFormatter.Deserialize(memoryStream);
        }
    }

    public static Dictionary<T, int> ToCountedDictionary<T>(this IEnumerable<T> items)
    {
        var dic = new Dictionary<T, int>();
        foreach (var item in items)
        {
            if (dic.ContainsKey(item))
                dic[item]++;
            else
                dic.Add(item, 1);
        }
        return dic;
    }
    public static Dictionary<TKey, TValue> KeyValuePairListToDictionary<TKey, TValue>(this IEnumerable<KeyValuePair<TKey, TValue>> pairs)
    {
        var dic = new Dictionary<TKey, TValue>();
        foreach (var item in pairs)
        {
            dic.Add(item.Key, item.Value);
        }
        return dic;
    }
    public class Scanner
    {
        private TextReader reader;
        public Scanner()
        {
            reader = Console.In;
        }
        public Scanner(TextReader reader)
        {
            this.reader = reader;
        }
        private string NextWord()
        {
            var sb = new StringBuilder();
            while (true)
            {
                var c = reader.Read();
                if ((c == 13 || c == 10 || c == 32) && sb.Length > 0 || c == -1)
                {
                    break;
                }
                sb.Append((char)c);
            }
            return sb.ToString().Trim();
        }
        public int NextInt() => NextWord().ToInt32();
        public string Next() => NextWord();
        public double NextDouble() => NextWord().ToDouble();
        public long NextLong() => NextWord().ToInt64();
        public BigInteger NextBigInteger() => NextWord().ToBigInteger();
        public string NextLine()
        {
            var str = reader.ReadLine();
            if (string.IsNullOrWhiteSpace(str))
            {
                return reader.ReadLine();
            }
            else
            {
                return str;
            }
        }
        private List<string> NextLineToList() => NextLine().Split(new char[] { ' ' },
                                                                  StringSplitOptions.RemoveEmptyEntries)
                                                           .ToList();
        public List<int> NextLineToIntList() => NextLineToList().Select(x => int.Parse(x))
                                                                .ToList();
    }
    public static long Sums(IEnumerable<int> list)
    {
        var sum = 0l;
        foreach (var item in list)
        {
            sum += Math.Abs(item);
        }
        return sum;
    }
    public static string IEnumerableListToString<T>(this IEnumerable<T> ls, bool withSpace = false)
    {
        var sb = new StringBuilder();
        foreach (var item in ls)
        {
            sb.Append(item);
            if (withSpace)
            {
                sb.Append(" ");
            }
        }
        return sb.ToString();
    }
    #endregion

    public static int[] dx = { -1, 1, 0, 0 };
    public static int[] dy = { 0, 0, 1, -1 };

    public static int[] dxx = { 0, 1, 1, 1, 0, -1, -1, -1 };
    public static int[] dyy = { 1, 1, 0, -1, -1, -1, 0, 1 };

}