Problem p03681 (C++)

Buggy submission (status: Runtime Error):
// C++ includes used for precompiling -*- C++ -*-

// Copyright (C) 2003-2013 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.

// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

/** @file stdc++.h
 *  This is an implementation file for a precompiled header.
 */

// 17.4.1.2 Headers

// C
// デバッグ用オプション：-fsanitize=undefined,address

// コンパイラ最適化
#pragma GCC optimize("Ofast")
#ifndef _GLIBCXX_NO_ASSERT
#include <cassert>
#endif
#include <cctype>
#include <cerrno>
#include <cfloat>
#include <ciso646>
#include <climits>
#include <clocale>
#include <cmath>
#include <csetjmp>
#include <csignal>
#include <cstdarg>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>

#if __cplusplus >= 201103L
#include <ccomplex>
#include <cfenv>
#include <cinttypes>
#include <cstdbool>
#include <cstdint>
#include <ctgmath>
#include <cwchar>
#include <cwctype>
#endif

// C++
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>

#if __cplusplus >= 201103L
#include <array>
#include <atomic>
#include <chrono>
#include <condition_variable>
#include <forward_list>
#include <future>
#include <initializer_list>
#include <mutex>
#include <random>
#include <ratio>
#include <regex>
#include <scoped_allocator>
#include <system_error>
#include <thread>
#include <tuple>
#include <type_traits>
#include <typeindex>
#include <unordered_map>
#include <unordered_set>
#endif

/*全てのライブラリをインクルード*/
using namespace std;
using ll = long long;
// マクロ
// forループ
// 引数は、(ループ内変数,動く範囲)か(ループ内変数,始めの数,終わりの数)、のどちらか
// Dがついてないものはループ変数は1ずつインクリメントされ、Dがついてるものはループ変数は1ずつデクリメントされる
// FORAは範囲for文(使いにくかったら消す)
#define rep(i, n) for (ll i = 0; i < ll(n); i++)
#define REPD(i, n) for (ll i = n - 1; i >= 0; i--)
#define FOR(i, a, b) for (ll i = a; i <= ll(b); i++)
#define FORD(i, a, b) for (ll i = a; i >= ll(b); i--)
#define FORA(i, I) for (const auto &i : I)
// xにはvectorなどのコンテナ
#define ALL(x) x.begin(), x.end()
#define SIZE(x) ll(x.size())
// 定数
#define INF 1000000000000 // 10^12:∞
#define MOD 1000000007    // 10^9+7:合同式の法
#define MAXR 100000       // 10^5:配列の最大のrange
// 略記
#define PB push_back // 挿入
#define MP make_pair // pairのコンストラクタ
#define F first      // pairの一つ目の要素
#define S second     // pairの二つ目の要素

ll func(int s) {
  if (s == 1)
    return 1;
  return s * func(s - 1) % MOD;
}

signed main() {
  // 入力の高速化用のコード
  // ios::sync_with_stdio(false);
  // cin.tie(nullptr);
  int n, m;
  cin >> n >> m;
  if (n < m) {
    int t = m;
    m = n;
    n = t;
  }
  if (n - m > 1) {
    cout << 0 << endl;
    return 0;
  }
  ll ans;
  if (n - m == 0) {
    ans = func(n) * func(m - 1) % MOD * m % MOD * 2 % MOD;
  } else if (n - m == 1) {
    ans = func(n) * func(m) % MOD;
  }
  cout << ans << endl;
  return 0;
}

Fixed submission:
// C++ includes used for precompiling -*- C++ -*-

// Copyright (C) 2003-2013 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.

// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

/** @file stdc++.h
 *  This is an implementation file for a precompiled header.
 */

// 17.4.1.2 Headers

// C
// デバッグ用オプション：-fsanitize=undefined,address

// コンパイラ最適化
#pragma GCC optimize("Ofast")
#ifndef _GLIBCXX_NO_ASSERT
#include <cassert>
#endif
#include <cctype>
#include <cerrno>
#include <cfloat>
#include <ciso646>
#include <climits>
#include <clocale>
#include <cmath>
#include <csetjmp>
#include <csignal>
#include <cstdarg>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>

#if __cplusplus >= 201103L
#include <ccomplex>
#include <cfenv>
#include <cinttypes>
#include <cstdbool>
#include <cstdint>
#include <ctgmath>
#include <cwchar>
#include <cwctype>
#endif

// C++
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>

#if __cplusplus >= 201103L
#include <array>
#include <atomic>
#include <chrono>
#include <condition_variable>
#include <forward_list>
#include <future>
#include <initializer_list>
#include <mutex>
#include <random>
#include <ratio>
#include <regex>
#include <scoped_allocator>
#include <system_error>
#include <thread>
#include <tuple>
#include <type_traits>
#include <typeindex>
#include <unordered_map>
#include <unordered_set>
#endif

/*全てのライブラリをインクルード*/
using namespace std;
using ll = long long;
// マクロ
// forループ
// 引数は、(ループ内変数,動く範囲)か(ループ内変数,始めの数,終わりの数)、のどちらか
// Dがついてないものはループ変数は1ずつインクリメントされ、Dがついてるものはループ変数は1ずつデクリメントされる
// FORAは範囲for文(使いにくかったら消す)
#define rep(i, n) for (ll i = 0; i < ll(n); i++)
#define REPD(i, n) for (ll i = n - 1; i >= 0; i--)
#define FOR(i, a, b) for (ll i = a; i <= ll(b); i++)
#define FORD(i, a, b) for (ll i = a; i >= ll(b); i--)
#define FORA(i, I) for (const auto &i : I)
// xにはvectorなどのコンテナ
#define ALL(x) x.begin(), x.end()
#define SIZE(x) ll(x.size())
// 定数
#define INF 1000000000000 // 10^12:∞
#define MOD 1000000007    // 10^9+7:合同式の法
#define MAXR 100000       // 10^5:配列の最大のrange
// 略記
#define PB push_back // 挿入
#define MP make_pair // pairのコンストラクタ
#define F first      // pairの一つ目の要素
#define S second     // pairの二つ目の要素

ll func(int s) {
  if (s == 1 || s == 0)
    return 1;
  return s * func(s - 1) % MOD;
}

signed main() {
  // 入力の高速化用のコード
  // ios::sync_with_stdio(false);
  // cin.tie(nullptr);
  int n, m;
  cin >> n >> m;
  if (n < m) {
    int t = m;
    m = n;
    n = t;
  }
  if (n - m > 1) {
    cout << 0 << endl;
    return 0;
  }
  ll ans;
  if (n - m == 0) {
    ans = func(n) * func(m - 1) % MOD * m % MOD * 2 % MOD;
  } else if (n - m == 1) {
    ans = func(n) * func(m) % MOD;
  }
  cout << ans << endl;
  return 0;
}