Problem p02840 (C++)

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

#define INF_LL (int64)1e18
#define INF (int32)1e9
#define REP(i, n) for (int64 i = 0; i < (n); i++)
#define FOR(i, a, b) for (int64 i = (a); i < (b); i++)
#define all(x) x.begin(), x.end()
#define fs first
#define sc second

using int32 = int_fast32_t;
using uint32 = uint_fast32_t;
using int64 = int_fast64_t;
using uint64 = uint_fast64_t;
using PII = pair<int32, int32>;
using PLL = pair<int64, int64>;

const double eps = 1e-10;

template <typename A, typename B> inline void chmin(A &a, B b) {
  if (a > b)
    a = b;
}
template <typename A, typename B> inline void chmax(A &a, B b) {
  if (a < b)
    a = b;
}

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...));
}

template <typename T, typename U, typename... V>
typename enable_if<is_same<T, U>::value != 0>::type fill_v(U &u, const V... v) {
  u = U(v...);
}

template <typename T, typename U, typename... V>
typename enable_if<is_same<T, U>::value == 0>::type fill_v(U &u, const V... v) {
  for (auto &e : u)
    fill_v<T>(e, v...);
}

template <::std::uint_fast64_t mod> class ModInt {
private:
  using value_type = ::std::uint_fast64_t;
  value_type n;

public:
  ModInt() : n(0) {}
  ModInt(value_type n_) : n(n_ % mod) {}
  ModInt(const ModInt &m) : n(m.n) {}

  template <typename T> explicit operator T() const {
    return static_cast<T>(n);
  }
  value_type get() const { return n; }

  friend ::std::ostream &operator<<(::std::ostream &os, const ModInt<mod> &a) {
    return os << a.n;
  }

  friend ::std::istream &operator>>(::std::istream &is, ModInt<mod> &a) {
    value_type x;
    is >> x;
    a = ModInt<mod>(x);
    return is;
  }

  bool operator==(const ModInt &m) const { return n == m.n; }
  bool operator!=(const ModInt &m) const { return n != m.n; }
  ModInt &operator*=(const ModInt &m) {
    n = n * m.n % mod;
    return *this;
  }

  ModInt pow(value_type b) const {
    ModInt ans = 1, m = ModInt(*this);
    while (b) {
      if (b & 1)
        ans *= m;
      m *= m;
      b >>= 1;
    }
    return ans;
  }

  ModInt inv() const { return (*this).pow(mod - 2); }
  ModInt &operator+=(const ModInt &m) {
    n += m.n;
    n = (n < mod ? n : n - mod);
    return *this;
  }
  ModInt &operator-=(const ModInt &m) {
    n += mod - m.n;
    n = (n < mod ? n : n - mod);
    return *this;
  }
  ModInt &operator/=(const ModInt &m) {
    *this *= m.inv();
    return *this;
  }
  ModInt operator+(const ModInt &m) const { return ModInt(*this) += m; }
  ModInt operator-(const ModInt &m) const { return ModInt(*this) -= m; }
  ModInt operator*(const ModInt &m) const { return ModInt(*this) *= m; }
  ModInt operator/(const ModInt &m) const { return ModInt(*this) /= m; }
  ModInt &operator++() {
    n += 1;
    return *this;
  }
  ModInt &operator--() {
    n -= 1;
    return *this;
  }
  ModInt operator++(int) {
    ModInt old(n);
    n += 1;
    return old;
  }
  ModInt operator--(int) {
    ModInt old(n);
    n -= 1;
    return old;
  }
  ModInt operator-() const { return ModInt(mod - n); }
};

constexpr int64 mod = 1e9 + 7;
using Mint = ModInt<mod>;

int main(void) {
  int64 N, X, D, aD;
  cin >> N >> X >> D;
  //  X= abs(X); D= abs(D);
  if (D == 0) {
    if (X == 0)
      return 1;
    else
      cout << N + 1 << endl;
    return 0;
  }
  aD = abs(D);
  map<int64, vector<PII>> mp;
  REP(i, N + 1) { // Sに選ぶ要素の数
    int64 diff_X = i * X - (N - i) * X;
    int64 mod_dx = (diff_X % aD + aD) % aD;
    int64 mini_D = i > 0 ? (i - 1) * (i) / 2 : 0;
    int64 maxi_D = (N) * (N - 1) / 2 - (N - i - 1) * (N - i) / 2;
    int64 mini_ot = (N) * (N - 1) / 2 - mini_D;
    int64 maxi_ot = (N) * (N - 1) / 2 - maxi_D;
    int64 ll = diff_X + D * (mini_D - mini_ot),
          rr = diff_X + D * (maxi_D - maxi_ot);
    mp[(ll % (aD * 2) + (aD * 2)) % (aD * 2)].emplace_back(minmax(
        diff_X + D * (mini_D - mini_ot), diff_X + D * (maxi_D - maxi_ot)));
  }
  int64 res = 0;
  for (auto &v : mp) {
    int64 l = numeric_limits<int64_t>::min();
    sort(all(v.sc));
    REP(i, v.sc.size()) {
      PII p(v.sc[i]);
      //      cout << p.fs << " " << p.sc << " " << l[0] << " " << l[1] << endl;
      if (l >= p.sc)
        continue;
      if (l < p.fs)
        res++;
      chmax(l, p.fs);
      res += (p.sc - l) / (aD * 2);
      chmax(l, p.sc);
    }
  }
  cout << res << endl;
}


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

#define INF_LL (int64)1e18
#define INF (int32)1e9
#define REP(i, n) for (int64 i = 0; i < (n); i++)
#define FOR(i, a, b) for (int64 i = (a); i < (b); i++)
#define all(x) x.begin(), x.end()
#define fs first
#define sc second

using int32 = int_fast32_t;
using uint32 = uint_fast32_t;
using int64 = int_fast64_t;
using uint64 = uint_fast64_t;
using PII = pair<int32, int32>;
using PLL = pair<int64, int64>;

const double eps = 1e-10;

template <typename A, typename B> inline void chmin(A &a, B b) {
  if (a > b)
    a = b;
}
template <typename A, typename B> inline void chmax(A &a, B b) {
  if (a < b)
    a = b;
}

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...));
}

template <typename T, typename U, typename... V>
typename enable_if<is_same<T, U>::value != 0>::type fill_v(U &u, const V... v) {
  u = U(v...);
}

template <typename T, typename U, typename... V>
typename enable_if<is_same<T, U>::value == 0>::type fill_v(U &u, const V... v) {
  for (auto &e : u)
    fill_v<T>(e, v...);
}

template <::std::uint_fast64_t mod> class ModInt {
private:
  using value_type = ::std::uint_fast64_t;
  value_type n;

public:
  ModInt() : n(0) {}
  ModInt(value_type n_) : n(n_ % mod) {}
  ModInt(const ModInt &m) : n(m.n) {}

  template <typename T> explicit operator T() const {
    return static_cast<T>(n);
  }
  value_type get() const { return n; }

  friend ::std::ostream &operator<<(::std::ostream &os, const ModInt<mod> &a) {
    return os << a.n;
  }

  friend ::std::istream &operator>>(::std::istream &is, ModInt<mod> &a) {
    value_type x;
    is >> x;
    a = ModInt<mod>(x);
    return is;
  }

  bool operator==(const ModInt &m) const { return n == m.n; }
  bool operator!=(const ModInt &m) const { return n != m.n; }
  ModInt &operator*=(const ModInt &m) {
    n = n * m.n % mod;
    return *this;
  }

  ModInt pow(value_type b) const {
    ModInt ans = 1, m = ModInt(*this);
    while (b) {
      if (b & 1)
        ans *= m;
      m *= m;
      b >>= 1;
    }
    return ans;
  }

  ModInt inv() const { return (*this).pow(mod - 2); }
  ModInt &operator+=(const ModInt &m) {
    n += m.n;
    n = (n < mod ? n : n - mod);
    return *this;
  }
  ModInt &operator-=(const ModInt &m) {
    n += mod - m.n;
    n = (n < mod ? n : n - mod);
    return *this;
  }
  ModInt &operator/=(const ModInt &m) {
    *this *= m.inv();
    return *this;
  }
  ModInt operator+(const ModInt &m) const { return ModInt(*this) += m; }
  ModInt operator-(const ModInt &m) const { return ModInt(*this) -= m; }
  ModInt operator*(const ModInt &m) const { return ModInt(*this) *= m; }
  ModInt operator/(const ModInt &m) const { return ModInt(*this) /= m; }
  ModInt &operator++() {
    n += 1;
    return *this;
  }
  ModInt &operator--() {
    n -= 1;
    return *this;
  }
  ModInt operator++(int) {
    ModInt old(n);
    n += 1;
    return old;
  }
  ModInt operator--(int) {
    ModInt old(n);
    n -= 1;
    return old;
  }
  ModInt operator-() const { return ModInt(mod - n); }
};

constexpr int64 mod = 1e9 + 7;
using Mint = ModInt<mod>;

int main(void) {
  int64 N, X, D, aD;
  cin >> N >> X >> D;
  //  X= abs(X); D= abs(D);
  if (D == 0) {
    if (X == 0)
      cout << 1 << endl;
    else
      cout << N + 1 << endl;
    return 0;
  }
  aD = abs(D);
  map<int64, vector<PII>> mp;
  REP(i, N + 1) { // Sに選ぶ要素の数
    int64 diff_X = i * X - (N - i) * X;
    int64 mod_dx = (diff_X % aD + aD) % aD;
    int64 mini_D = i > 0 ? (i - 1) * (i) / 2 : 0;
    int64 maxi_D = (N) * (N - 1) / 2 - (N - i - 1) * (N - i) / 2;
    int64 mini_ot = (N) * (N - 1) / 2 - mini_D;
    int64 maxi_ot = (N) * (N - 1) / 2 - maxi_D;
    int64 ll = diff_X + D * (mini_D - mini_ot),
          rr = diff_X + D * (maxi_D - maxi_ot);
    mp[(ll % (aD * 2) + (aD * 2)) % (aD * 2)].emplace_back(minmax(
        diff_X + D * (mini_D - mini_ot), diff_X + D * (maxi_D - maxi_ot)));
  }
  int64 res = 0;
  for (auto &v : mp) {
    int64 l = numeric_limits<int64_t>::min();
    sort(all(v.sc));
    REP(i, v.sc.size()) {
      PII p(v.sc[i]);
      //      cout << p.fs << " " << p.sc << " " << l[0] << " " << l[1] << endl;
      if (l >= p.sc)
        continue;
      if (l < p.fs)
        res++;
      chmax(l, p.fs);
      res += (p.sc - l) / (aD * 2);
      chmax(l, p.sc);
    }
  }
  cout << res << endl;
}