Problem p02840 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
using LL = long long;

#define fs first
#define sc second

const LL MOD = 1e9 + 7;
const double EPS = 1e-10;

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

// std::vector Declaration
template <typename T> vector<T> make_v(size_t a) { return vector<T>(a); }
template <typename T, typename... Ts> auto make_v(size_t a, Ts... ts) {
  return vector<decltype(make_v<T>(ts...))>(a, make_v<T>(ts...));
}

// std::vector Declaration and Initialization
template <typename T> vector<T> make_vector(size_t a, T x) {
  return vector<T>(a, x);
}
template <typename T, typename U, typename... Ts>
auto make_vector(size_t a, U b, Ts... ts) {
  return vector<decltype(make_vector<T>(b, ts...))>(a,
                                                    make_vector<T>(b, ts...));
}

// std::vector Input
template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (auto &e : v) {
    is >> e;
  }
  return is;
}

// std::vector Debug
template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
  os << "[";
  bool a = 1;
  for (auto e : v) {
    os << (a ? "" : " ");
    os << e;
    a = 0;
  }
  os << "]";
  return os;
}

// std::array Debug
template <typename T, size_t n>
ostream &operator<<(ostream &os, const array<T, n> &v) {
  os << "[";
  bool a = 1;
  for (auto e : v) {
    os << (a ? "" : " ");
    os << e;
    a = 0;
  }
  os << "]";
  return os;
}

// std::deque Debug
template <typename T> ostream &operator<<(ostream &os, const deque<T> &d) {
  os << "[";
  bool a = 1;
  for (auto e : d) {
    os << (a ? "" : " ");
    os << e;
    a = 0;
  }
  os << "]";
  return os;
}

// std::pair Debug
template <typename T, typename U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
  os << "(" << p.first << " " << p.second << ")";
  return os;
}

// std::set Debug
template <typename T> ostream &operator<<(ostream &os, const set<T> &st) {
  os << "{";
  bool a = 1;
  for (auto e : st) {
    os << (a ? "" : " ");
    os << e;
    a = 0;
  }
  os << "}";
  return os;
}

// std::multiset Debug
template <typename T> ostream &operator<<(ostream &os, const multiset<T> &st) {
  os << "{";
  bool a = 1;
  for (auto e : st) {
    os << (a ? "" : " ");
    os << e;
    a = 0;
  }
  os << "}";
  return os;
}

// std::map Debug
template <typename T, typename U>
ostream &operator<<(ostream &os, const map<T, U> &mp) {
  os << "{";
  bool a = 1;
  for (auto e : mp) {
    os << (a ? "" : " ");
    os << e.first << ":" << e.second;
    a = 0;
  }
  os << "}";
  return os;
}

// std::tuple Debug
template <int N, class Tuple> void out(ostream &os, const Tuple &t) {}
template <int N, class Tuple, class H, class... Ts>
void out(ostream &os, const Tuple &t) {
  if (N)
    os << " ";
  os << get<N>(t);
  out<N + 1, Tuple, Ts...>(os, t);
}
template <class... Ts> ostream &operator<<(ostream &os, const tuple<Ts...> &t) {
  os << "(";
  out<0, tuple<Ts...>, Ts...>(os, t);
  os << ")";
  return os;
}

// Debug
#define DUMP(x) cerr << #x << " = " << (x) << endl

// Weighted edge
template <typename T> struct edge {
  int src, to;
  T cost;

  edge(int to, T cost) : src(-1), to(to), cost(cost) {}
  edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}

  edge &operator=(const int &x) {
    to = x;
    return *this;
  }

  operator int() const { return to; }

  friend ostream &operator<<(ostream &os, const edge &e) {
    return os << "(" << e.src << "->" << e.to << ":" << e.cost << ")";
  }
};

template <typename T> using Edges = vector<edge<T>>;
template <typename T> using WeightedGraph = vector<Edges<T>>;
using UnWeightedGraph = vector<vector<int>>;
template <typename T> using MatrixGraph = vector<vector<T>>;

int main() {
  LL N, X, D;
  cin >> N >> X >> D;

  LL gcd = __gcd(D, X);
  D /= gcd;
  X /= gcd;

  if (D < 0) {
    D *= -1;
    X *= -1;
  }

  LL ans = 0;
  for (LL r = 0; r < D && r <= N; ++r) {
    multiset<pair<LL, int>> event;
    for (LL i = r; i <= N; i += D) {
      LL m = i / D;
      event.emplace(i * (i - 1) / 2 + X * m, 0);
      event.emplace(i * (N - 1) - i * (i - 1) / 2 + 1 + X * m, 1);
    }
    LL INF = numeric_limits<LL>::max();
    LL prv = INF, cnt = 0, sum = 0;
    for (auto &tp : event) {
      assert(cnt >= 0);
      if (cnt && prv != INF) {
        sum += tp.fs - prv;
      }
      prv = tp.fs;
      cnt += tp.sc ? -1 : 1;
    }
    ans += sum;
  }

  cout << ans << endl;

  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
using LL = long long;

#define fs first
#define sc second

const LL MOD = 1e9 + 7;
const double EPS = 1e-10;

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

// std::vector Declaration
template <typename T> vector<T> make_v(size_t a) { return vector<T>(a); }
template <typename T, typename... Ts> auto make_v(size_t a, Ts... ts) {
  return vector<decltype(make_v<T>(ts...))>(a, make_v<T>(ts...));
}

// std::vector Declaration and Initialization
template <typename T> vector<T> make_vector(size_t a, T x) {
  return vector<T>(a, x);
}
template <typename T, typename U, typename... Ts>
auto make_vector(size_t a, U b, Ts... ts) {
  return vector<decltype(make_vector<T>(b, ts...))>(a,
                                                    make_vector<T>(b, ts...));
}

// std::vector Input
template <typename T> istream &operator>>(istream &is, vector<T> &v) {
  for (auto &e : v) {
    is >> e;
  }
  return is;
}

// std::vector Debug
template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
  os << "[";
  bool a = 1;
  for (auto e : v) {
    os << (a ? "" : " ");
    os << e;
    a = 0;
  }
  os << "]";
  return os;
}

// std::array Debug
template <typename T, size_t n>
ostream &operator<<(ostream &os, const array<T, n> &v) {
  os << "[";
  bool a = 1;
  for (auto e : v) {
    os << (a ? "" : " ");
    os << e;
    a = 0;
  }
  os << "]";
  return os;
}

// std::deque Debug
template <typename T> ostream &operator<<(ostream &os, const deque<T> &d) {
  os << "[";
  bool a = 1;
  for (auto e : d) {
    os << (a ? "" : " ");
    os << e;
    a = 0;
  }
  os << "]";
  return os;
}

// std::pair Debug
template <typename T, typename U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
  os << "(" << p.first << " " << p.second << ")";
  return os;
}

// std::set Debug
template <typename T> ostream &operator<<(ostream &os, const set<T> &st) {
  os << "{";
  bool a = 1;
  for (auto e : st) {
    os << (a ? "" : " ");
    os << e;
    a = 0;
  }
  os << "}";
  return os;
}

// std::multiset Debug
template <typename T> ostream &operator<<(ostream &os, const multiset<T> &st) {
  os << "{";
  bool a = 1;
  for (auto e : st) {
    os << (a ? "" : " ");
    os << e;
    a = 0;
  }
  os << "}";
  return os;
}

// std::map Debug
template <typename T, typename U>
ostream &operator<<(ostream &os, const map<T, U> &mp) {
  os << "{";
  bool a = 1;
  for (auto e : mp) {
    os << (a ? "" : " ");
    os << e.first << ":" << e.second;
    a = 0;
  }
  os << "}";
  return os;
}

// std::tuple Debug
template <int N, class Tuple> void out(ostream &os, const Tuple &t) {}
template <int N, class Tuple, class H, class... Ts>
void out(ostream &os, const Tuple &t) {
  if (N)
    os << " ";
  os << get<N>(t);
  out<N + 1, Tuple, Ts...>(os, t);
}
template <class... Ts> ostream &operator<<(ostream &os, const tuple<Ts...> &t) {
  os << "(";
  out<0, tuple<Ts...>, Ts...>(os, t);
  os << ")";
  return os;
}

// Debug
#define DUMP(x) cerr << #x << " = " << (x) << endl

// Weighted edge
template <typename T> struct edge {
  int src, to;
  T cost;

  edge(int to, T cost) : src(-1), to(to), cost(cost) {}
  edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}

  edge &operator=(const int &x) {
    to = x;
    return *this;
  }

  operator int() const { return to; }

  friend ostream &operator<<(ostream &os, const edge &e) {
    return os << "(" << e.src << "->" << e.to << ":" << e.cost << ")";
  }
};

template <typename T> using Edges = vector<edge<T>>;
template <typename T> using WeightedGraph = vector<Edges<T>>;
using UnWeightedGraph = vector<vector<int>>;
template <typename T> using MatrixGraph = vector<vector<T>>;

int main() {
  LL N, X, D;
  cin >> N >> X >> D;

  if (D == 0) {
    if (X == 0) {
      cout << 1 << endl;
    } else {
      cout << N + 1 << endl;
    }
    return 0;
  }

  LL gcd = __gcd(D, X);
  D /= gcd;
  X /= gcd;

  if (D < 0) {
    D *= -1;
    X *= -1;
  }

  LL ans = 0;
  for (LL r = 0; r < D && r <= N; ++r) {
    multiset<pair<LL, int>> event;
    for (LL i = r; i <= N; i += D) {
      LL m = i / D;
      event.emplace(i * (i - 1) / 2 + X * m, 0);
      event.emplace(i * (N - 1) - i * (i - 1) / 2 + 1 + X * m, 1);
    }
    LL INF = numeric_limits<LL>::max();
    LL prv = INF, cnt = 0, sum = 0;
    for (auto &tp : event) {
      assert(cnt >= 0);
      if (cnt && prv != INF) {
        sum += tp.fs - prv;
      }
      prv = tp.fs;
      cnt += tp.sc ? -1 : 1;
    }
    ans += sum;
  }

  cout << ans << endl;

  return 0;
}