Problem p02840 (C++)

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

// macros
#define pb emplace_back
#define mk make_pair
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define rep(i, n) FOR(i, 0, n)
#define rrep(i, n) for (int i = ((int)(n)-1); i >= 0; i--)
#define irep(itr, st) for (auto itr = (st).begin(); itr != (st).end(); ++itr)
#define irrep(itr, st) for (auto itr = (st).rbegin(); itr != (st).rend(); ++itr)
#define all(x) (x).begin(), (x).end()
#define sz(x) ((int)(x).size())
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end())
#define bit(n) (1LL << (n))
// functions
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 (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T> istream &operator>>(istream &is, vector<T> &vec) {
  for (auto &v : vec)
    is >> v;
  return is;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  for (int i = 0; i < vec.size(); i++) {
    os << vec[i];
    if (i + 1 != vec.size())
      os << " ";
  }
  return os;
}
template <typename T> ostream &operator<<(ostream &os, const set<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end())
      os << " ";
  }
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const unordered_set<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end())
      os << " ";
  }
  return os;
}
template <typename T> ostream &operator<<(ostream &os, const multiset<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end())
      os << " ";
  }
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const unordered_multiset<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end())
      os << " ";
  }
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  os << p.first << " " << p.second;
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const map<T1, T2> &mp) {
  for (auto itr = mp.begin(); itr != mp.end(); ++itr) {
    os << itr->first << ":" << itr->second;
    auto titr = itr;
    if (++titr != mp.end())
      os << " ";
  }
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const unordered_map<T1, T2> &mp) {
  for (auto itr = mp.begin(); itr != mp.end(); ++itr) {
    os << itr->first << ":" << itr->second;
    auto titr = itr;
    if (++titr != mp.end())
      os << " ";
  }
  return os;
}
//  types
using ll = long long int;
using P = pair<int, int>;
// constants
const int inf = 1e9;
const ll linf = 1LL << 50;
const double EPS = 1e-10;
const int mod = 1000000007;
const int dx[4] = {-1, 0, 1, 0};
const int dy[4] = {0, -1, 0, 1};
// io
struct fast_io {
  fast_io() {
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout << fixed << setprecision(20);
  }
} fast_io_;

template <typename T> struct MergableRangeSet {
  using PT = pair<T, T>;
  set<PT> st;
  MergableRangeSet() {}
  // [l, r)
  void add(T l, T r) {
    if (r <= l)
      return;
    auto itr = st.upper_bound(make_pair(l, 0));
    if (itr != st.begin()) {
      auto litr = itr;
      --litr;
      if (litr->second >= r) {
        return;
      } else if (litr->second >= l) {
        l = litr->first;
        st.erase(litr);
      }
    }
    if (itr != st.end()) {
      if (r < itr->first)
        st.insert(make_pair(l, r));
      else {
        PT p = make_pair(l, max(r, itr->second));
        st.erase(itr);
        st.insert(p);
      }
    } else {
      st.insert(make_pair(l, r));
    }
  }
  typename set<PT>::iterator begin() noexcept { return st.begin(); }
  typename set<PT>::iterator end() noexcept { return st.end(); }
  size_t size() { return st.size(); }
};

map<ll, MergableRangeSet<ll>> ms;

int main(int argc, char const *argv[]) {
  int n;
  cin >> n;
  ll x, d;
  cin >> x >> d;
  rep(i, n + 1) {
    ll b = ((ll)i * x) % d;
    ll of = ((ll)i * x) / d;
    if (ms.find(b) == ms.end())
      ms[b] = MergableRangeSet<ll>();
    ll l = (ll)(i - 1) * (ll)i / 2;
    ll r = l + (ll)(i) * (ll)(n - i);
    ms[b].add(l + of, r + of + 1);
  }
  ll res = 0;
  irep(itr, ms) {
    auto &v = itr->second;
    irep(im, v) { res += im->second - im->first; }
  }
  cout << res << endl;
  return 0;
}


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

// macros
#define pb emplace_back
#define mk make_pair
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define rep(i, n) FOR(i, 0, n)
#define rrep(i, n) for (int i = ((int)(n)-1); i >= 0; i--)
#define irep(itr, st) for (auto itr = (st).begin(); itr != (st).end(); ++itr)
#define irrep(itr, st) for (auto itr = (st).rbegin(); itr != (st).rend(); ++itr)
#define all(x) (x).begin(), (x).end()
#define sz(x) ((int)(x).size())
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end())
#define bit(n) (1LL << (n))
// functions
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 (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T> istream &operator>>(istream &is, vector<T> &vec) {
  for (auto &v : vec)
    is >> v;
  return is;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  for (int i = 0; i < vec.size(); i++) {
    os << vec[i];
    if (i + 1 != vec.size())
      os << " ";
  }
  return os;
}
template <typename T> ostream &operator<<(ostream &os, const set<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end())
      os << " ";
  }
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const unordered_set<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end())
      os << " ";
  }
  return os;
}
template <typename T> ostream &operator<<(ostream &os, const multiset<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end())
      os << " ";
  }
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const unordered_multiset<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end())
      os << " ";
  }
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  os << p.first << " " << p.second;
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const map<T1, T2> &mp) {
  for (auto itr = mp.begin(); itr != mp.end(); ++itr) {
    os << itr->first << ":" << itr->second;
    auto titr = itr;
    if (++titr != mp.end())
      os << " ";
  }
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const unordered_map<T1, T2> &mp) {
  for (auto itr = mp.begin(); itr != mp.end(); ++itr) {
    os << itr->first << ":" << itr->second;
    auto titr = itr;
    if (++titr != mp.end())
      os << " ";
  }
  return os;
}
//  types
using ll = long long int;
using P = pair<int, int>;
// constants
const int inf = 1e9;
const ll linf = 1LL << 50;
const double EPS = 1e-10;
const int mod = 1000000007;
const int dx[4] = {-1, 0, 1, 0};
const int dy[4] = {0, -1, 0, 1};
// io
struct fast_io {
  fast_io() {
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout << fixed << setprecision(20);
  }
} fast_io_;

template <typename T> struct MergableRangeSet {
  using PT = pair<T, T>;
  set<PT> st;
  MergableRangeSet() {}
  // [l, r)
  void add(T l, T r) {
    if (r <= l)
      return;
    auto itr = st.upper_bound(make_pair(l, 0));
    if (itr != st.begin()) {
      auto litr = itr;
      --litr;
      if (litr->second >= r) {
        return;
      } else if (litr->second >= l) {
        l = litr->first;
        st.erase(litr);
      }
    }
    if (itr != st.end()) {
      if (r < itr->first)
        st.insert(make_pair(l, r));
      else {
        PT p = make_pair(l, max(r, itr->second));
        st.erase(itr);
        st.insert(p);
      }
    } else {
      st.insert(make_pair(l, r));
    }
  }
  typename set<PT>::iterator begin() noexcept { return st.begin(); }
  typename set<PT>::iterator end() noexcept { return st.end(); }
  size_t size() { return st.size(); }
};

map<ll, MergableRangeSet<ll>> ms;

int main(int argc, char const *argv[]) {
  int n;
  cin >> n;
  ll x, d;
  cin >> x >> d;
  if (d == 0) {
    set<ll> st;
    rep(i, n + 1) st.insert((ll)i * x);
    cout << sz(st) << endl;
    return 0;
  }
  rep(i, n + 1) {
    ll b = ((ll)i * x) % d;
    ll of = ((ll)i * x) / d;
    if (ms.find(b) == ms.end())
      ms[b] = MergableRangeSet<ll>();
    ll l = (ll)(i - 1) * (ll)i / 2;
    ll r = l + (ll)(i) * (ll)(n - i);
    ms[b].add(l + of, r + of + 1);
  }
  ll res = 0;
  irep(itr, ms) {
    auto &v = itr->second;
    irep(im, v) { res += im->second - im->first; }
  }
  cout << res << endl;
  return 0;
}