Problem:
Score : 300 points

Problem Statement

Snuke has N dogs and M monkeys. He wants them to line up in a row.

As a Japanese saying goes, these dogs and monkeys are on bad terms. ("ken'en no naka", literally "the relationship of dogs and monkeys", means a relationship of mutual hatred.) Snuke is trying to reconsile them, by arranging the animals so that there are neither two adjacent dogs nor two adjacent monkeys.

How many such arrangements there are? Find the count modulo 10^9+7 (since animals cannot understand numbers larger than that).
Here, dogs and monkeys are both distinguishable. Also, two arrangements that result from reversing each other are distinguished.

Constraints

1 ≤ N,M ≤ 10^5

Input

Input is given from Standard Input in the following format:

N M

Output

Print the number of possible arrangements, modulo 10^9+7.

Sample Input 1

2 2

Sample Output 1

8

We will denote the dogs by A and B, and the monkeys by C and D. There are eight possible arrangements: ACBD, ADBC, BCAD, BDAC, CADB, CBDA, DACB and DBCA.

Sample Input 2

3 2

Sample Output 2

12

Sample Input 3

1 8

Sample Output 3

0

Sample Input 4

100000 100000

Sample Output 4

530123477

Solution in C# (Mono 4.6.2.0) (judge verdict: Wrong Answer)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Numerics;
using static System.Math;
using static AtCoderTemplate.MyExtensions;
using static AtCoderTemplate.MyInputOutputs;
using static AtCoderTemplate.MyNumericFunctions;
using static AtCoderTemplate.MyAlgorithm;

namespace AtCoderTemplate {
    public class Program {
        public static void Main (string[] args) {
            var NM = ReadInts ();
            var N = NM[0];
            var M = NM[1];
            var quot = new BigInteger ((Pow (10, 9) + 7));
            if (Abs (N - M) == 0) {
                var fact = BigFact (N);
                Print ((2 * fact * fact) % quot);
            } else if (Abs (N - M) == 1) {
                var fact1 = BigFact (Min (N, M));
                var fact2 = fact1 * Min (N, M);
                Print ((fact1 * fact2) % quot);
            } else {
                Print (0);
            }
        }

        public static BigInteger BigFact (int n) {
            return Enumerable.Range (1, n).Aggregate (new BigInteger (1), (a, b) => BigInteger.Multiply (a, b));
        }
    }

    public static class MyInputOutputs {
        /* Input & Output*/
        public static int ReadInt () {
            return int.Parse (Console.ReadLine ());
        }
        public static long ReadLong () {
            return long.Parse (Console.ReadLine ());
        }
        public static List<int> ReadInts () {
            return Console.ReadLine ().Split (' ').Select (c => int.Parse (c)).ToList ();
        }
        public static List<long> ReadLongs () {
            return Console.ReadLine ().Split (' ').Select (c => long.Parse (c)).ToList ();
        }
        public static List<List<int>> ReadIntColumns (int n) {
            /*
            入力例
            A1 B1
            A2 B2
            ...
            An Bn

            出力例
            [[A1,A2,...,An], [B1,B2,...,Bn]]
            */
            var rows = Enumerable.Range (0, n).Select (i => ReadInts ()).ToList ();
            var m = rows.FirstOrDefault ()?.Count () ?? 0;
            return Enumerable.Range (0, m).Select (i => rows.Select (items => items[i]).ToList ()).ToList ();
        }
        public static List<List<long>> ReadLongColumns (int n) {
            /*
            入力例
            A1 B1
            A2 B2
            ...
            An Bn

            出力例
            [[A1,A2,...,An], [B1,B2,...,Bn]]
            */
            var rows = Enumerable.Range (0, n).Select (i => ReadLongs ()).ToList ();
            var m = rows.FirstOrDefault ()?.Count () ?? 0;
            return Enumerable.Range (0, m).Select (i => rows.Select (items => items[i]).ToList ()).ToList ();
        }

        public static void Print<T> (T item) {
            Console.WriteLine (item);
        }
        public static void PrintIf<T1, T2> (bool condition, T1 trueResult, T2 falseResult) {
            if (condition) {
                Console.WriteLine (trueResult);
            } else {
                Console.WriteLine (falseResult);
            }
        }

        public static void PrintRow<T> (IEnumerable<T> list) {
            /* 横ベクトルで表示
            A B C D ...
            */
            if (!list.IsEmpty ()) {
                Console.Write (list.First ());
                foreach (var item in list.Skip (1)) {
                    Console.Write ($" {item}");
                }
            }
            Console.Write ("\n");
        }
        public static void PrintColomn<T> (IEnumerable<T> list) {
            /* 縦ベクトルで表示
            A
            B
            C
            D
            ...
            */
            foreach (var item in list) {
                Console.WriteLine (item);
            }
        }
        public static void Print2DArray<T> (IEnumerable<IEnumerable<T>> sources) {
            foreach (var row in sources) {
                PrintRow (row);
            }
        }
    }

    public static class MyNumericFunctions {
        public static bool isEven (int a) {
            return a % 2 == 0;
        }
        public static bool isEven (long a) {
            return a % 2 == 0;
        }
        public static bool isOdd (int a) {
            return !isEven (a);
        }
        public static bool isOdd (long a) {
            return !isEven (a);
        }

        public static long nPk (int n, int k) {
            if (k > n) {
                return 0;
            } else {
                return Enumerable.Range (n - k + 1, k).Aggregate ((long) 1, ((i, m) => i * m));
            }
        }
        public static long Fact (int n) {
            return nPk (n, n);
        }
        public static long nCk (int n, int k) {
            if (k > n) {
                return 0;
            } else {
                return nPk (n, k) / Fact (k);
            }
        }
        // 最大公約数
        public static long GCD (long m, long n) {
            if (m <= 0 || n <= 0) throw new ArgumentException ();

            if (m < n) return GCD (n, m);
            while (n != 0) {
                var n2 = m % n;
                m = n;
                n = n2;
            }
            return m;
        }
        // 最小公倍数
        public static long LCM (long m, long n) {
            var ans = checked ((long) (BigInteger.Multiply (m, n) / GCD (m, n)));
            return ans;
        }
    }

    public static class MyAlgorithm {
        /// <summary>
        /// 二分探索法
        /// O(log N)
        /// </summary>
        /// <param name="list">探索するリスト</param>
        /// <param name="predicate">条件の述語関数</param>
        /// <param name="ng">条件を満たさない既知のindex</param>
        /// <param name="ok">条件を満たす既知のindex</param>
        /// <typeparam name="T">順序関係を持つ型(IComparableを実装する)</typeparam>
        /// <returns>条件を満たすindexの内、境界に最も近いものを返す</returns>
        public static int BinarySearch<T> (IList<T> list, Func<T, bool> predicate, int ng, int ok)
        where T : IComparable<T> {
            while (Abs (ok - ng) > 1) {
                int mid = (ok + ng) / 2;
                if (predicate (list[mid])) {
                    ok = mid;
                } else {
                    ng = mid;
                }
            }
            return ok;
        }

        /// <summary>
        /// 辺の集まりを操作するオブジェクト
        /// </summary>
        public class Edge {
            long[, ] edge;
            public int NodeNum { get; }
            public Edge (int nodeNum, long overDistance) {
                var edge = new long[nodeNum, nodeNum];
                foreach (var i in Enumerable.Range (0, nodeNum)) {
                    foreach (var j in Enumerable.Range (0, nodeNum)) {
                        if (i != j) {
                            edge[i, j] = overDistance;
                        } else {
                            edge[i, j] = 0;
                        }
                    }
                }

                this.edge = edge;
                this.NodeNum = nodeNum;
            }
            public Edge (Edge edge) {
                this.edge = new long[edge.NodeNum, edge.NodeNum];
                foreach (var i in Enumerable.Range (0, edge.NodeNum)) {
                    foreach (var j in Enumerable.Range (0, edge.NodeNum)) {
                        this.edge[i, j] = edge.GetLength (i, j);
                    }
                }
                this.NodeNum = edge.NodeNum;
            }

            public List<List<long>> ToList () {
                return Enumerable.Range (0, NodeNum).Select (i =>
                    Enumerable.Range (0, NodeNum).Select (j =>
                        edge[i, j]
                    ).ToList ()
                ).ToList ();
            }

            public void Add (int node1, int node2, long distance) {
                edge[node1, node2] = distance;
            }

            public long GetLength (int node1, int node2) {
                return edge[node1, node2];
            }
        }

        /// <summary>
        /// ワーシャルフロイド法
        /// O(N^3)
        /// </summary>
        /// <param name="edge">Edgeオブジェクト</param>
        /// <param name="nodeNum">ノードの数</param>
        /// <returns>各ノード間の最短距離を辺として持つEdgeオブジェクト</returns>
        public static Edge WarshallFloyd (Edge edge) {
            var res = new Edge (edge);
            foreach (var b in Enumerable.Range (0, edge.NodeNum)) {
                foreach (var a in Enumerable.Range (0, edge.NodeNum)) {
                    foreach (var c in Enumerable.Range (0, edge.NodeNum)) {
                        res.Add (a, c, Min (res.GetLength (a, c), res.GetLength (a, b) + res.GetLength (b, c)));
                    }
                }
            }
            return res;
        }
    }

    public static class MyExtensions {
        // AppendとPrependが、.NET Standard 1.6からの追加で、Mono 4.6.2 はそれに対応して仕様はあるが、実装がない
        public static IEnumerable<T> Append<T> (this IEnumerable<T> source, T element) {
            return source.Concat (Enumerable.Repeat (element, 1));
        }

        public static IEnumerable<T> Prepend<T> (this IEnumerable<T> source, T element) {
            return Enumerable.Repeat (element, 1).Concat (source);
        }

        // TakeLastとSkipLastが、.Net Standard 2.1からの追加で、Mono 4.6.2 はそれに対応していない
        public static IEnumerable<T> TakeLast<T> (this IEnumerable<T> source, int count) {
            return source.Skip (source.Count () - count);
        }

        public static IEnumerable<T> SkipLast<T> (this IEnumerable<T> source, int count) {
            return source.Take (source.Count () - count);
        }

        public static bool IsEmpty<T> (this IEnumerable<T> source) {
            return !source.Any ();
        }

        /// <summary>
        /// インデックスiの位置の要素からk個取り除く
        /// O(N)
        /// </summary>
        public static IEnumerable<T> TakeAwayRange<T> (this IEnumerable<T> source, int i, int count) {
            return source.Take (i).Concat (source.Skip (i + count));
        }

        /// <summary>
        /// インデックスiの位置の要素を取り除く
        /// O(N)
        /// </summary>
        public static IEnumerable<T> TakeAwayAt<T> (this IEnumerable<T> source, int i) {
            return source.TakeAwayRange (i, 1);
        }

        /// <summary>
        /// インデックスiの位置にシーケンスを挿入する
        /// O(N + K)
        /// </summary>
        public static IEnumerable<T> InsertEnumAt<T> (this IEnumerable<T> source, int i, IEnumerable<T> inserted) {
            return source.Take (i).Concat (inserted).Concat (source.Skip (i));
        }

        /// <summary>
        /// 順列を得る
        /// O(N!)
        /// </summary>
        public static IEnumerable<IEnumerable<T>> Perm<T> (this IEnumerable<T> source, int n) {
            if (n == 0 || source.IsEmpty () || source.Count () < n) {
                return Enumerable.Empty<IEnumerable<T>> ();
            } else if (n == 1) {
                return source.Select (i => new List<T> { i });
            } else {
                var nexts = source.Select ((x, i) =>
                    new { next = source.Take (i).Concat (source.Skip (i + 1)), selected = source.Take (i + 1).Last () });
                return nexts.SelectMany (next => Perm (next.next, n - 1).Select (item => item.Prepend (next.selected)));
            }
        }

        /// <summary>
        /// シーケンスの隣り合う要素を2引数の関数に適用したシーケンスを得る
        /// </summary>
        /// <para>O(N)</para>
        /// <param name="source">元のシーケンス</param>
        /// <param name="func">2引数関数</param>
        /// <example>[1,2,3,4].MapAdjacent(f) => [f(1,2), f(2,3), f(3,4)]</example>
        public static IEnumerable<TR> MapAdjacent<T1, TR> (this IEnumerable<T1> source, Func<T1, T1, TR> func) {
            var list = source.ToList ();
            return Enumerable.Range (1, list.Count - 1)
                .Select (i => func (list[i], list[i - 1]));
        }

        /// <summary>
        /// 累積項を要素にもつシーケンスを得る(初項は、first)
        /// <para>O(N)</para>
        /// </summary>
        /// <param name="source">元のシーケンス</param>
        /// <param name="func">2引数関数f</param>
        /// <param name="first">func(first, source[0])のための初項</param>
        /// <example> [1,2,3].Scanl1(f,0) => [0, f(0,1), f(f(0,1),2), f(f(f(0,1),2),3)]</example>
        public static IEnumerable<TR> Scanl<T, TR> (this IEnumerable<T> source, TR first, Func<TR, T, TR> func) {
            var list = source.ToList ();
            var result = new List<TR> { first };
            foreach (var i in Enumerable.Range (0, source.Count ())) {
                result.Add (func (result[i], list[i]));
            }
            return result;
        }
        /// <summary>
        /// 累積項を要素にもつシーケンスを得る（初項は、source.First()）
        /// <para>O(N)</para>
        /// </summary>
        /// <param name="source">元のシーケンス</param>
        /// <param name="func">2引数関数f</param>
        /// <example> [1,2,3].Scanl1(f) => [1, f(1,2), f(f(1,2),3)]</example>
        public static IEnumerable<T> Scanl1<T> (this IEnumerable<T> source, Func<T, T, T> func) {
            var list = source.ToList ();
            var result = new List<T> { list[0] };
            foreach (var i in Enumerable.Range (1, source.Count () - 1)) {
                result.Add (func (result[i - 1], list[i]));
            }
            return result;
        }

        /// <summary>
        /// 昇順にソートしたインデックスを得る
        /// </summary>
        /// <para>O(N * log N)</para>
        public static IEnumerable<int> SortIndex<T> (this IEnumerable<T> source) {
            return source
                .Select ((item, i) => new { Item = item, Index = i })
                .OrderBy (x => x.Item)
                .Select (x => x.Index);
        }
    }
}