Problem p02840 (C++)

Buggy submission (status: Runtime Error):
#ifdef LOCAL
#define _GLIBCXX_DEBUG
#endif
#include <bits/stdc++.h>
using namespace std;
#define rep(i, s, t) for (ll i = (ll)(s); i < (ll)(t); i++)
#define rrep(i, s, t) for (ll i = (ll)(s - 1); (ll)(t) <= i; i--)
#define all(x) (x).begin(), (x).end()
typedef long long ll;
typedef long double ld;
typedef pair<ll, ll> Pll;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<vvl> vvvl;
constexpr ll INF = numeric_limits<ll>::max() / 4;
constexpr ll n_max = 2e5 + 10;
#define int ll

template <typename A, typename B> string to_string(pair<A, B> p);
string to_string(const string &s) { return '"' + s + '"'; }
string to_string(const char *c) { return to_string((string)c); }
string to_string(bool b) { return (b ? "true" : "false"); }
template <size_t N> string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++)
    res += static_cast<char>('0' + v[i]);
  return res;
}
template <typename A> string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first)
      res += ", ";
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}
template <typename A, typename B> string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

void debug_out() { cerr << endl; }
template <typename Head, typename... Tail> void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}

#ifdef LOCAL
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif

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

signed main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  ll n, x, d;
  cin >> n >> x >> d;
  if (d == 0) {
    assert(false);
    cout << n + 1 << endl;
    return 0;
  }
  map<ll, set<Pll>> mp;
  ll min_, max_;
  rep(i, 0, n + 1) {
    min_ = x * i + d * (i * (i - 1) / 2);
    max_ = x * i + d * ((2 * n - i - 1) * i / 2);
    debug(min_, max_);
    max_ += d;
    if (min_ > max_)
      swap(min_, max_);
    debug(min_, max_);
    ll m = min_ % d;
    if (m < 0)
      m += d;
    if (mp[m].empty()) {
      mp[m].insert({min_, max_});
      continue;
    }
    auto itr = mp[m].lower_bound({min_, -INF});
    ll e = min_;
    if (itr != mp[m].begin()) {
      itr--;
      if (itr->second >= min_) {
        min_ = itr->first;
        e = itr->second;
        auto itr2 = itr;
        itr++;
        mp[m].erase(itr2);
      } else
        itr++;
    }
    bool is_end = false;
    while (e < max_) {
      if (itr == mp[m].end()) {
        is_end = true;
        break;
      }
      if (itr->first > max_) {
        e = max_;
        break;
      }
      e = itr->second;
      auto itr2 = itr;
      itr++;
      mp[m].erase(itr2);
    }

    if (is_end) {
      e = max_;
    }

    mp[m].insert({min_, e});
  }

  ll ans = 0;
  for (auto &st : mp) {
    debug(st.first);
    for (auto &p : st.second) {
      debug(p);
      ans += (p.second - p.first) / abs(d);
    }
  }

  cout << ans << endl;
}

Fixed submission:
#ifdef LOCAL
#define _GLIBCXX_DEBUG
#endif
#include <bits/stdc++.h>
using namespace std;
#define rep(i, s, t) for (ll i = (ll)(s); i < (ll)(t); i++)
#define rrep(i, s, t) for (ll i = (ll)(s - 1); (ll)(t) <= i; i--)
#define all(x) (x).begin(), (x).end()
typedef long long ll;
typedef long double ld;
typedef pair<ll, ll> Pll;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<vvl> vvvl;
constexpr ll INF = numeric_limits<ll>::max() / 4;
constexpr ll n_max = 2e5 + 10;
#define int ll

template <typename A, typename B> string to_string(pair<A, B> p);
string to_string(const string &s) { return '"' + s + '"'; }
string to_string(const char *c) { return to_string((string)c); }
string to_string(bool b) { return (b ? "true" : "false"); }
template <size_t N> string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++)
    res += static_cast<char>('0' + v[i]);
  return res;
}
template <typename A> string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first)
      res += ", ";
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}
template <typename A, typename B> string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

void debug_out() { cerr << endl; }
template <typename Head, typename... Tail> void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}

#ifdef LOCAL
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif

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

signed main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  ll n, x, d;
  cin >> n >> x >> d;
  if (d == 0) {
    if (x)
      cout << n + 1 << endl;
    else
      cout << 1 << endl;
    return 0;
  }
  map<ll, set<Pll>> mp;
  ll min_, max_;
  rep(i, 0, n + 1) {
    min_ = x * i + d * (i * (i - 1) / 2);
    max_ = x * i + d * ((2 * n - i - 1) * i / 2);
    debug(min_, max_);
    max_ += d;
    if (min_ > max_)
      swap(min_, max_);
    debug(min_, max_);
    ll m = min_ % d;
    if (m < 0)
      m += d;
    if (mp[m].empty()) {
      mp[m].insert({min_, max_});
      continue;
    }
    auto itr = mp[m].lower_bound({min_, -INF});
    ll e = min_;
    if (itr != mp[m].begin()) {
      itr--;
      if (itr->second >= min_) {
        min_ = itr->first;
        e = itr->second;
        auto itr2 = itr;
        itr++;
        mp[m].erase(itr2);
      } else
        itr++;
    }
    bool is_end = false;
    while (e < max_) {
      if (itr == mp[m].end()) {
        is_end = true;
        break;
      }
      if (itr->first > max_) {
        e = max_;
        break;
      }
      e = itr->second;
      auto itr2 = itr;
      itr++;
      mp[m].erase(itr2);
    }

    if (is_end) {
      e = max_;
    }

    mp[m].insert({min_, e});
  }

  ll ans = 0;
  for (auto &st : mp) {
    debug(st.first);
    for (auto &p : st.second) {
      debug(p);
      ans += (p.second - p.first) / abs(d);
    }
  }

  cout << ans << endl;
}