Problem p02974 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#define FOR(i, a, n) for (ll i = (ll)a; i < (ll)n; i++)
#define FORR(i, n) for (ll i = (ll)n - 1LL; i >= 0LL; i--)
#define rep(i, n) FOR(i, 0, n)
#define ALL(x) ((x).begin(), (x).end())
using namespace std;
using ll = long long;
template <typename T> using V = vector<T>;

constexpr int Mod = 998244353;
constexpr int mod = 1e9 + 7;
constexpr ll inf = 1LL << 60;

template <typename T> constexpr bool chmax(T &a, const T &b) {
  if (a >= b)
    return false;
  a = b;
  return true;
}
template <typename T> constexpr bool chmin(T &a, const T &b) {
  if (a <= b)
    return false;
  a = b;
  return true;
}

/*-------------------------------------------*/

template <int M> class ModInt {
  int x;

public:
  constexpr ModInt() : x(0) {}
  constexpr ModInt(int64_t y) : x(y >= 0 ? y % M : (M - (-y) % M) % M) {}
  constexpr ModInt &operator+=(const ModInt &p) {
    if ((x += p.x) >= M)
      x -= M;
    return *this;
  }
  constexpr ModInt &operator-=(const ModInt &p) {
    if ((x += M - p.x) >= M)
      x -= M;
    return *this;
  }
  constexpr ModInt &operator*=(const ModInt &p) {
    x = (int)(1LL * x * p.x % M);
    return *this;
  }
  constexpr ModInt &operator/=(const ModInt &p) {
    *this *= p.inverse();
    return *this;
  }
  constexpr ModInt operator-() const { return ModInt(-x); }
  constexpr ModInt operator+(const ModInt &p) const {
    return ModInt(*this) += p;
  }
  constexpr ModInt operator-(const ModInt &p) const {
    return ModInt(*this) -= p;
  }
  constexpr ModInt operator*(const ModInt &p) const {
    return ModInt(*this) *= p;
  }
  constexpr ModInt operator/(const ModInt &p) const {
    return ModInt(*this) /= p;
  }
  constexpr bool operator==(const ModInt &p) const { return x == p.x; }
  constexpr bool operator!=(const ModInt &p) const { return x != p.x; }
  constexpr ModInt inverse() const {
    int a = x, b = M, u = 1, v = 0, t = 0;
    while (b > 0) {
      t = a / b;
      swap(a -= t * b, b);
      swap(u -= t * v, v);
    }
    return ModInt(u);
  }
  constexpr ModInt pow(const int64_t &n) const {
    ModInt ret(1), mul(x);
    int64_t k = n % (M - 1);
    if (k < 0)
      k += M - 1;
    while (k > 0) {
      if (k & 1)
        ret *= mul;
      mul *= mul;
      k >>= 1;
    }
    return ret;
  }
  constexpr friend ostream &operator<<(ostream &os, const ModInt &p) {
    return os << p.x;
  }
  constexpr friend istream &operator>>(istream &is, ModInt &a) {
    int64_t t = 0;
    is >> t;
    a = ModInt(t);
    return (is);
  }
};
using modint = ModInt<mod>;

int main() {
  cin.tie(0);
  ios::sync_with_stdio(0);

  int n, k;
  cin >> n >> k;
  V<V<V<modint>>> dp(n + 1, V<V<modint>>(n, V<modint>(k + 1)));
  dp[0][0][0] = 1;
  rep(i, n) rep(j, n) FOR(l, j * 2, k + 1) {
    dp[i + 1][j][l] = dp[i][j][l - 2 * j] * (2 * j + 1);
    dp[i + 1][j][l] += dp[i][j + 1][l - 2 * j] * (j + 1) * (j + 1);
    if (j)
      dp[i + 1][j][l] += dp[i][j - 1][l - 2 * j];
  }
  cout << dp[n][0][k] << endl;

  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
#define FOR(i, a, n) for (ll i = (ll)a; i < (ll)n; i++)
#define FORR(i, n) for (ll i = (ll)n - 1LL; i >= 0LL; i--)
#define rep(i, n) FOR(i, 0, n)
#define ALL(x) ((x).begin(), (x).end())
using namespace std;
using ll = long long;
template <typename T> using V = vector<T>;

constexpr int Mod = 998244353;
constexpr int mod = 1e9 + 7;
constexpr ll inf = 1LL << 60;

template <typename T> constexpr bool chmax(T &a, const T &b) {
  if (a >= b)
    return false;
  a = b;
  return true;
}
template <typename T> constexpr bool chmin(T &a, const T &b) {
  if (a <= b)
    return false;
  a = b;
  return true;
}

/*-------------------------------------------*/

template <int M> class ModInt {
  int x;

public:
  constexpr ModInt() : x(0) {}
  constexpr ModInt(int64_t y) : x(y >= 0 ? y % M : (M - (-y) % M) % M) {}
  constexpr ModInt &operator+=(const ModInt &p) {
    if ((x += p.x) >= M)
      x -= M;
    return *this;
  }
  constexpr ModInt &operator-=(const ModInt &p) {
    if ((x += M - p.x) >= M)
      x -= M;
    return *this;
  }
  constexpr ModInt &operator*=(const ModInt &p) {
    x = (int)(1LL * x * p.x % M);
    return *this;
  }
  constexpr ModInt &operator/=(const ModInt &p) {
    *this *= p.inverse();
    return *this;
  }
  constexpr ModInt operator-() const { return ModInt(-x); }
  constexpr ModInt operator+(const ModInt &p) const {
    return ModInt(*this) += p;
  }
  constexpr ModInt operator-(const ModInt &p) const {
    return ModInt(*this) -= p;
  }
  constexpr ModInt operator*(const ModInt &p) const {
    return ModInt(*this) *= p;
  }
  constexpr ModInt operator/(const ModInt &p) const {
    return ModInt(*this) /= p;
  }
  constexpr bool operator==(const ModInt &p) const { return x == p.x; }
  constexpr bool operator!=(const ModInt &p) const { return x != p.x; }
  constexpr ModInt inverse() const {
    int a = x, b = M, u = 1, v = 0, t = 0;
    while (b > 0) {
      t = a / b;
      swap(a -= t * b, b);
      swap(u -= t * v, v);
    }
    return ModInt(u);
  }
  constexpr ModInt pow(const int64_t &n) const {
    ModInt ret(1), mul(x);
    int64_t k = n % (M - 1);
    if (k < 0)
      k += M - 1;
    while (k > 0) {
      if (k & 1)
        ret *= mul;
      mul *= mul;
      k >>= 1;
    }
    return ret;
  }
  constexpr friend ostream &operator<<(ostream &os, const ModInt &p) {
    return os << p.x;
  }
  constexpr friend istream &operator>>(istream &is, ModInt &a) {
    int64_t t = 0;
    is >> t;
    a = ModInt(t);
    return (is);
  }
};
using modint = ModInt<mod>;

int main() {
  cin.tie(0);
  ios::sync_with_stdio(0);

  int n, k;
  cin >> n >> k;
  V<V<V<modint>>> dp(n + 1, V<V<modint>>(n + 1, V<modint>(k + 1)));
  dp[0][0][0] = 1;
  rep(i, n) rep(j, n) FOR(l, j * 2, k + 1) {
    dp[i + 1][j][l] = dp[i][j][l - 2 * j] * (2 * j + 1);
    dp[i + 1][j][l] += dp[i][j + 1][l - 2 * j] * (j + 1) * (j + 1);
    if (j)
      dp[i + 1][j][l] += dp[i][j - 1][l - 2 * j];
  }
  cout << dp[n][0][k] << endl;

  return 0;
}