Problem p02634 (C++) — buggy submission, verdict: Runtime Error

#include "bits/stdc++.h"
#include <random>
using namespace std;
typedef long long int lint;
typedef pair<lint, lint> plint;
typedef pair<double long, double long> pld;
#define ALL(x) (x).begin(), (x).end()
#define SZ(x) ((lint)(x).size())
#define POW2(n) (1LL << (n))
#define FOR(i, begin, end)                                                     \\
  for (lint i = (begin), i##_end_ = (end); i < i##_end_; i++)
#define IFOR(i, begin, end)                                                    \\
  for (lint i = (end)-1, i##_begin_ = (begin); i >= i##_begin_; i--)
#define REP(i, n) FOR(i, 0, n)
#define IREP(i, n) IFOR(i, 0, n)
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T1, typename T2>
pair<T1, T2> operator+(const pair<T1, T2> &l, const pair<T1, T2> &r) {
  return make_pair(l.first + r.first, l.second + r.second);
}
template <typename T1, typename T2>
pair<T1, T2> operator-(const pair<T1, T2> &l, const pair<T1, T2> &r) {
  return make_pair(l.first - r.first, l.second - r.second);
}
const lint MOD = 998244353, INF = 1e16;

template <std::int_fast64_t Modulus> class modint {
  using u64 = std::int_fast64_t;

public:
  u64 a;
  constexpr modint(const u64 x = 0) noexcept : a(x % Modulus) {}
  constexpr u64 &value() noexcept { return a; }
  constexpr const u64 &value() const noexcept { return a; }
  constexpr modint operator+(const modint rhs) const noexcept {
    return modint(*this) += rhs;
  }
  constexpr modint operator-(const modint rhs) const noexcept {
    return modint(*this) -= rhs;
  }
  constexpr modint operator*(const modint rhs) const noexcept {
    return modint(*this) *= rhs;
  }
  constexpr modint operator/(const modint rhs) const noexcept {
    return modint(*this) /= rhs;
  }
  constexpr modint &operator+=(const modint rhs) noexcept {
    a += rhs.a;
    if (a >= Modulus) {
      a -= Modulus;
    }
    return *this;
  }
  constexpr modint &operator-=(const modint rhs) noexcept {
    if (a < rhs.a) {
      a += Modulus;
    }
    a -= rhs.a;
    return *this;
  }
  constexpr modint &operator*=(const modint rhs) noexcept {
    a = a * rhs.a % Modulus;
    return *this;
  }
  constexpr modint &operator/=(modint rhs) noexcept {
    u64 exp = Modulus - 2;
    while (exp) {
      if (exp % 2) {
        *this *= rhs;
      }
      rhs *= rhs;
      exp /= 2;
    }
    return *this;
  }
};
typedef modint<MOD> ModInt;

ModInt mod_pow(ModInt x, ModInt n) {
  ModInt ret = 1;
  while (n.a > 0) {
    if (n.a & 1)
      (ret *= x);
    (x *= x);
    n.a >>= 1;
  }
  return ret;
}

lint A, B, C, D;
ModInt dp[3003][3003];
int main() {
  cin >> A >> B >> C >> D;
  dp[A][B] = 1;
  FOR(i, A, C + 1) {
    FOR(j, B, D + 1) {
      dp[i + 1][j] += dp[i][j] * j;
      if (dp[i][j + 1].a != 0)
        dp[i][j + 1] += dp[i][j] * i - dp[i - 1][j] * j * (i - 1);
      else
        dp[i][j + 1] += dp[i][j] * i;
    }
  }
  cout << dp[C][D].a << endl;
}


Error: 0

Fixed submission:

#include "bits/stdc++.h"
#include <random>
using namespace std;
typedef long long int lint;
typedef pair<lint, lint> plint;
typedef pair<double long, double long> pld;
#define ALL(x) (x).begin(), (x).end()
#define SZ(x) ((lint)(x).size())
#define POW2(n) (1LL << (n))
#define FOR(i, begin, end)                                                     \\
  for (lint i = (begin), i##_end_ = (end); i < i##_end_; i++)
#define IFOR(i, begin, end)                                                    \\
  for (lint i = (end)-1, i##_begin_ = (begin); i >= i##_begin_; i--)
#define REP(i, n) FOR(i, 0, n)
#define IREP(i, n) IFOR(i, 0, n)
template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T1, typename T2>
pair<T1, T2> operator+(const pair<T1, T2> &l, const pair<T1, T2> &r) {
  return make_pair(l.first + r.first, l.second + r.second);
}
template <typename T1, typename T2>
pair<T1, T2> operator-(const pair<T1, T2> &l, const pair<T1, T2> &r) {
  return make_pair(l.first - r.first, l.second - r.second);
}
const lint MOD = 998244353, INF = 1e16;

template <std::int_fast64_t Modulus> class modint {
  using u64 = std::int_fast64_t;

public:
  u64 a;
  constexpr modint(const u64 x = 0) noexcept : a(x % Modulus) {}
  constexpr u64 &value() noexcept { return a; }
  constexpr const u64 &value() const noexcept { return a; }
  constexpr modint operator+(const modint rhs) const noexcept {
    return modint(*this) += rhs;
  }
  constexpr modint operator-(const modint rhs) const noexcept {
    return modint(*this) -= rhs;
  }
  constexpr modint operator*(const modint rhs) const noexcept {
    return modint(*this) *= rhs;
  }
  constexpr modint operator/(const modint rhs) const noexcept {
    return modint(*this) /= rhs;
  }
  constexpr modint &operator+=(const modint rhs) noexcept {
    a += rhs.a;
    if (a >= Modulus) {
      a -= Modulus;
    }
    return *this;
  }
  constexpr modint &operator-=(const modint rhs) noexcept {
    if (a < rhs.a) {
      a += Modulus;
    }
    a -= rhs.a;
    return *this;
  }
  constexpr modint &operator*=(const modint rhs) noexcept {
    a = a * rhs.a % Modulus;
    return *this;
  }
  constexpr modint &operator/=(modint rhs) noexcept {
    u64 exp = Modulus - 2;
    while (exp) {
      if (exp % 2) {
        *this *= rhs;
      }
      rhs *= rhs;
      exp /= 2;
    }
    return *this;
  }
};
typedef modint<MOD> ModInt;

ModInt mod_pow(ModInt x, ModInt n) {
  ModInt ret = 1;
  while (n.a > 0) {
    if (n.a & 1)
      (ret *= x);
    (x *= x);
    n.a >>= 1;
  }
  return ret;
}

lint A, B, C, D;
ModInt dp[3001][3001];
int main() {
  cin >> A >> B >> C >> D;
  dp[A][B] = 1;
  FOR(i, A, C + 1) {
    FOR(j, B, D + 1) {
      dp[i + 1][j] += dp[i][j] * j;
      if (dp[i][j + 1].a != 0)
        dp[i][j + 1] += dp[i][j] * i - dp[i - 1][j] * j * (i - 1);
      else
        dp[i][j + 1] += dp[i][j] * i;
    }
  }
  cout << dp[C][D].a << endl;
}