Problem p02974 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>

using namespace std;

#define int long long
// #define double long double
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define FORR(i, a, b) for (int i = (a); i > (b); --i)
#define REP(i, n) for (int i = 0; i < (n); ++i)
#define REPR(i, n) for (int i = n; i >= 0; i--)
#define FOREACH(x, a) for (auto &(x) : (a))
#define VECCIN(x)                                                              \
  for (auto &youso_ : (x))                                                     \
  cin >> youso_
#define bitcnt(x) __builtin_popcount(x)
#define lbit(x) __builtin_ffsll(x)
#define rbit(x) __builtin_clzll(x)
#define SZ(x) ((long long)(x).size())
#define fi first
#define se second
#define All(a) (a).begin(), (a).end()
#define rAll(a) (a).rbegin(), (a).rend()
#define cinfast() cin.tie(0), ios::sync_with_stdio(false)
#define PERM(c)                                                                \
  sort(All(c));                                                                \
  for (bool cp = true; cp; cp = next_permutation(All(c)))
#define MKORDER(n)                                                             \
  vector<int> od(n);                                                           \
  iota(All(od), 0LL);

template <typename T = long long> inline T IN() {
  T x;
  cin >> x;
  return (x);
}
inline void CIN() {}
template <class Head, class... Tail>
inline void CIN(Head &&head, Tail &&...tail) {
  cin >> head;
  CIN(move(tail)...);
}
#define CCIN(...)                                                              \
  char __VA_ARGS__;                                                            \
  CIN(__VA_ARGS__)
#define DCIN(...)                                                              \
  double __VA_ARGS__;                                                          \
  CIN(__VA_ARGS__)
#define LCIN(...)                                                              \
  long long __VA_ARGS__;                                                       \
  CIN(__VA_ARGS__)
#define SCIN(...)                                                              \
  string __VA_ARGS__;                                                          \
  CIN(__VA_ARGS__)
#define Yes(a) cout << (a ? "Yes" : "No") << "\n"
#define YES(a) cout << (a ? "YES" : "NO") << "\n"
#define Printv(v)                                                              \
  {                                                                            \
    FOREACH(x, v) { cout << x << " "; }                                        \
    cout << "\n";                                                              \
  }
template <typename T = string> inline void eputs(T s) {
  cout << s << "\n";
  exit(0);
}
template <typename A, size_t N, typename T>
void Fill(A (&array)[N], const T &val) {
  std::fill((T *)array, (T *)(array + N), val);
}

template <typename T> using PQG = priority_queue<T, vector<T>, greater<T>>;
template <typename T> using PQ = priority_queue<T>;

typedef long long ll;
typedef vector<ll> VL;
typedef vector<VL> VVL;
typedef pair<ll, ll> PL;
typedef vector<PL> VPL;
typedef vector<bool> VB;
typedef vector<double> VD;
typedef vector<string> VS;

const int INF = 1e9;
const int MOD = 1e9 + 7;
// const int MOD = 998244353;
const ll LINF = 1e18;
const double PI = atan(1.0) * 4.0;
const ll dw[] = {1, 1, 0, -1, -1, -1, 0, 1};
const ll dh[] = {0, 1, 1, 1, 0, -1, -1, -1};
#define PI 3.141592653589793238

// 1000000007 で割ったあまりを扱う構造体
template <int MOD> struct Fp {
  long long val;
  constexpr Fp(long long v = 0) noexcept : val(v % MOD) {
    if (val < 0)
      v += MOD;
  }
  constexpr int getmod() { return MOD; }
  constexpr Fp operator-() const noexcept { return val ? MOD - val : 0; }
  constexpr Fp operator+(const Fp &r) const noexcept { return Fp(*this) += r; }
  constexpr Fp operator-(const Fp &r) const noexcept { return Fp(*this) -= r; }
  constexpr Fp operator*(const Fp &r) const noexcept { return Fp(*this) *= r; }
  constexpr Fp operator/(const Fp &r) const noexcept { return Fp(*this) /= r; }
  constexpr Fp &operator+=(const Fp &r) noexcept {
    val += r.val;
    if (val >= MOD)
      val -= MOD;
    return *this;
  }
  constexpr Fp &operator-=(const Fp &r) noexcept {
    val -= r.val;
    if (val < 0)
      val += MOD;
    return *this;
  }
  constexpr Fp &operator*=(const Fp &r) noexcept {
    val = val * r.val % MOD;
    return *this;
  }
  constexpr Fp &operator/=(const Fp &r) noexcept {
    long long a = r.val, b = MOD, u = 1, v = 0;
    while (b) {
      long long t = a / b;
      a -= t * b;
      swap(a, b);
      u -= t * v;
      swap(u, v);
    }
    val = val * u % MOD;
    if (val < 0)
      val += MOD;
    return *this;
  }
  constexpr bool operator==(const Fp &r) const noexcept {
    return this->val == r.val;
  }
  constexpr bool operator!=(const Fp &r) const noexcept {
    return this->val != r.val;
  }
  friend constexpr ostream &operator<<(ostream &os, const Fp<MOD> &x) noexcept {
    return os << x.val;
  }
  friend constexpr istream &operator>>(istream &is, Fp<MOD> &x) noexcept {
    return is >> x.val;
  }
  friend constexpr Fp<MOD> modpow(const Fp<MOD> &a, long long n) noexcept {
    if (n == 0)
      return 1;
    auto t = modpow(a, n / 2);
    t = t * t;
    if (n & 1)
      t = t * a;
    return t;
  }
};

using mint = Fp<MOD>;

mint dp[2][51][2505];

signed main() {
  cinfast();
  LCIN(N, K);
  ll now = 0, next = 1;
  dp[now][0][0] = 1;
  REP(i, N) {
    Fill(dp[next], 0);
    REP(j, N + 1) FOR(k, 2 * j, K + 1) {
      // 両方保留
      if (j)
        dp[next][j][k] += dp[now][j - 1][k - 2 * j];
      // 保留箱に入れるかつ保留ボールを入れる
      dp[next][j][k] += (mint)(j + 1) * (j + 1) * dp[now][j + 1][k - 2 * j];
      // 保留箱に入れるまたは保留ボールを入れる
      dp[next][j][k] += (mint)(2 * j + 1) * dp[now][j][k - 2 * j];
    }
    swap(now, next);
  }
  cout << dp[now][0][K] << "\n";
}


Fixed submission:
#include <bits/stdc++.h>

using namespace std;

#define int long long
// #define double long double
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define FORR(i, a, b) for (int i = (a); i > (b); --i)
#define REP(i, n) for (int i = 0; i < (n); ++i)
#define REPR(i, n) for (int i = n; i >= 0; i--)
#define FOREACH(x, a) for (auto &(x) : (a))
#define VECCIN(x)                                                              \
  for (auto &youso_ : (x))                                                     \
  cin >> youso_
#define bitcnt(x) __builtin_popcount(x)
#define lbit(x) __builtin_ffsll(x)
#define rbit(x) __builtin_clzll(x)
#define SZ(x) ((long long)(x).size())
#define fi first
#define se second
#define All(a) (a).begin(), (a).end()
#define rAll(a) (a).rbegin(), (a).rend()
#define cinfast() cin.tie(0), ios::sync_with_stdio(false)
#define PERM(c)                                                                \
  sort(All(c));                                                                \
  for (bool cp = true; cp; cp = next_permutation(All(c)))
#define MKORDER(n)                                                             \
  vector<int> od(n);                                                           \
  iota(All(od), 0LL);

template <typename T = long long> inline T IN() {
  T x;
  cin >> x;
  return (x);
}
inline void CIN() {}
template <class Head, class... Tail>
inline void CIN(Head &&head, Tail &&...tail) {
  cin >> head;
  CIN(move(tail)...);
}
#define CCIN(...)                                                              \
  char __VA_ARGS__;                                                            \
  CIN(__VA_ARGS__)
#define DCIN(...)                                                              \
  double __VA_ARGS__;                                                          \
  CIN(__VA_ARGS__)
#define LCIN(...)                                                              \
  long long __VA_ARGS__;                                                       \
  CIN(__VA_ARGS__)
#define SCIN(...)                                                              \
  string __VA_ARGS__;                                                          \
  CIN(__VA_ARGS__)
#define Yes(a) cout << (a ? "Yes" : "No") << "\n"
#define YES(a) cout << (a ? "YES" : "NO") << "\n"
#define Printv(v)                                                              \
  {                                                                            \
    FOREACH(x, v) { cout << x << " "; }                                        \
    cout << "\n";                                                              \
  }
template <typename T = string> inline void eputs(T s) {
  cout << s << "\n";
  exit(0);
}
template <typename A, size_t N, typename T>
void Fill(A (&array)[N], const T &val) {
  std::fill((T *)array, (T *)(array + N), val);
}

template <typename T> using PQG = priority_queue<T, vector<T>, greater<T>>;
template <typename T> using PQ = priority_queue<T>;

typedef long long ll;
typedef vector<ll> VL;
typedef vector<VL> VVL;
typedef pair<ll, ll> PL;
typedef vector<PL> VPL;
typedef vector<bool> VB;
typedef vector<double> VD;
typedef vector<string> VS;

const int INF = 1e9;
const int MOD = 1e9 + 7;
// const int MOD = 998244353;
const ll LINF = 1e18;
const double PI = atan(1.0) * 4.0;
const ll dw[] = {1, 1, 0, -1, -1, -1, 0, 1};
const ll dh[] = {0, 1, 1, 1, 0, -1, -1, -1};
#define PI 3.141592653589793238

// 1000000007 で割ったあまりを扱う構造体
template <int MOD> struct Fp {
  long long val;
  constexpr Fp(long long v = 0) noexcept : val(v % MOD) {
    if (val < 0)
      v += MOD;
  }
  constexpr int getmod() { return MOD; }
  constexpr Fp operator-() const noexcept { return val ? MOD - val : 0; }
  constexpr Fp operator+(const Fp &r) const noexcept { return Fp(*this) += r; }
  constexpr Fp operator-(const Fp &r) const noexcept { return Fp(*this) -= r; }
  constexpr Fp operator*(const Fp &r) const noexcept { return Fp(*this) *= r; }
  constexpr Fp operator/(const Fp &r) const noexcept { return Fp(*this) /= r; }
  constexpr Fp &operator+=(const Fp &r) noexcept {
    val += r.val;
    if (val >= MOD)
      val -= MOD;
    return *this;
  }
  constexpr Fp &operator-=(const Fp &r) noexcept {
    val -= r.val;
    if (val < 0)
      val += MOD;
    return *this;
  }
  constexpr Fp &operator*=(const Fp &r) noexcept {
    val = val * r.val % MOD;
    return *this;
  }
  constexpr Fp &operator/=(const Fp &r) noexcept {
    long long a = r.val, b = MOD, u = 1, v = 0;
    while (b) {
      long long t = a / b;
      a -= t * b;
      swap(a, b);
      u -= t * v;
      swap(u, v);
    }
    val = val * u % MOD;
    if (val < 0)
      val += MOD;
    return *this;
  }
  constexpr bool operator==(const Fp &r) const noexcept {
    return this->val == r.val;
  }
  constexpr bool operator!=(const Fp &r) const noexcept {
    return this->val != r.val;
  }
  friend constexpr ostream &operator<<(ostream &os, const Fp<MOD> &x) noexcept {
    return os << x.val;
  }
  friend constexpr istream &operator>>(istream &is, Fp<MOD> &x) noexcept {
    return is >> x.val;
  }
  friend constexpr Fp<MOD> modpow(const Fp<MOD> &a, long long n) noexcept {
    if (n == 0)
      return 1;
    auto t = modpow(a, n / 2);
    t = t * t;
    if (n & 1)
      t = t * a;
    return t;
  }
};

using mint = Fp<MOD>;

mint dp[2][55][2555];

signed main() {
  cinfast();
  LCIN(N, K);
  ll now = 0, next = 1;
  dp[now][0][0] = 1;
  REP(i, N) {
    Fill(dp[next], 0);
    REP(j, N + 1) FOR(k, 2 * j, K + 1) {
      // 両方保留
      if (j)
        dp[next][j][k] += dp[now][j - 1][k - 2 * j];
      // 保留箱に入れるかつ保留ボールを入れる
      dp[next][j][k] += (mint)(j + 1) * (j + 1) * dp[now][j + 1][k - 2 * j];
      // 保留箱に入れるまたは保留ボールを入れる
      dp[next][j][k] += (mint)(2 * j + 1) * dp[now][j][k - 2 * j];
    }
    swap(now, next);
  }
  cout << dp[now][0][K] << "\n";
}