Problem p02840 (C++)

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

inline int toInt(string s) {
  int v;
  istringstream sin(s);
  sin >> v;
  return v;
}
template <class T> inline string toString(T x) {
  ostringstream sout;
  sout << x;
  return sout.str();
}

typedef long long ll;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<long long> VL;
typedef vector<vector<long long>> VVL;
typedef vector<string> VS;
typedef pair<ll, ll> P;
typedef tuple<int, int, int> tpl;

#define ALL(a) (a).begin(), (a).end()
#define SORT(c) sort((c).begin(), (c).end())
#define REVERSE(c) reverse((c).begin(), (c).end())
#define LB(a, x) lower_bound((a).begin(), (a).end(), x) - (a).begin()
#define UB(a, x) upper_bound((a).begin(), (a).end(), x) - (a).begin()

#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define REP(i, n) FOR(i, 0, n)
#define RFOR(i, a, b) for (int i = (a)-1; i >= (b); --i)
#define RREP(i, n) RFOR(i, n, 0)

#define en "\n"

constexpr double EPS = 1e-9;
constexpr double PI = 3.1415926535897932;
constexpr int INF = 2147483647;
constexpr long long LINF = 1LL << 60;
constexpr long long MOD = 1000000007; // 998244353;

#define dump(x) cerr << #x << " = " << (x) << endl;
#define debug(x)                                                               \
  cerr << #x << " = " << (x) << " (L" << __LINE__ << ")"                       \
       << " " << __FILE__ << endl;

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

template <class T> void remove_duplication(vector<T> &v) {
  sort(v.begin(), v.end());
  v.erase(unique(v.begin(), v.end()), v.end());
}

long long modp(long long a, long long p) {
  long long res = a % p;
  if (res < 0)
    res += p;
  return res;
}

void Main() {
  ll N, D, X;
  cin >> N >> X >> D;

  if (X == 0 && D == 0) {
    cout << 1 << en;
    return;
  }

  int flag = D > 0 ? 1 : -1;
  D = abs(D);

  VL mod{0};
  REP(i, N) {
    ll m = modp(mod.back() + X, D);
    mod.push_back(m);
  }
  remove_duplication(mod);
  map<ll, int> mp;
  REP(i, mod.size()) mp[mod[i]] = i;

  vector<vector<P>> v(mod.size());
  v[0].emplace_back(0, 0);
  ll m = 0;
  for (ll i = 1; i < N + 1; i++) {
    m = modp(m + X, D);
    int k = mp[m];
    ll mn = i * X / D + flag * i * (i - 1) / 2;
    ll mx = i * X / D + flag * (N * (N - 1) / 2 - (N - i) * (N - i - 1) / 2);
    if (flag < 0)
      swap(mn, mx);
    v[k].emplace_back(mn, mx);
  }

  ll ans = 0;
  REP(i, mp.size()) {
    SORT(v[i]);
    ll e = -LINF;
    REP(j, v[i].size()) {
      if (e < v[i][j].first)
        ans += v[i][j].second - v[i][j].first + 1;
      else if (e < v[i][j].second)
        ans += v[i][j].second - e;
      chmax(e, v[i][j].second);
    }
  }

  cout << ans << en;
  return;
}

int main(void) {
  cin.tie(0);
  ios_base::sync_with_stdio(0);
  cout << fixed << setprecision(15);
  int t = 1; // cin>>t;
  REP(_, t) { Main(); }
  return 0;
}

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

inline int toInt(string s) {
  int v;
  istringstream sin(s);
  sin >> v;
  return v;
}
template <class T> inline string toString(T x) {
  ostringstream sout;
  sout << x;
  return sout.str();
}

typedef long long ll;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<long long> VL;
typedef vector<vector<long long>> VVL;
typedef vector<string> VS;
typedef pair<ll, ll> P;
typedef tuple<int, int, int> tpl;

#define ALL(a) (a).begin(), (a).end()
#define SORT(c) sort((c).begin(), (c).end())
#define REVERSE(c) reverse((c).begin(), (c).end())
#define LB(a, x) lower_bound((a).begin(), (a).end(), x) - (a).begin()
#define UB(a, x) upper_bound((a).begin(), (a).end(), x) - (a).begin()

#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define REP(i, n) FOR(i, 0, n)
#define RFOR(i, a, b) for (int i = (a)-1; i >= (b); --i)
#define RREP(i, n) RFOR(i, n, 0)

#define en "\n"

constexpr double EPS = 1e-9;
constexpr double PI = 3.1415926535897932;
constexpr int INF = 2147483647;
constexpr long long LINF = 1LL << 60;
constexpr long long MOD = 1000000007; // 998244353;

#define dump(x) cerr << #x << " = " << (x) << endl;
#define debug(x)                                                               \
  cerr << #x << " = " << (x) << " (L" << __LINE__ << ")"                       \
       << " " << __FILE__ << endl;

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

template <class T> void remove_duplication(vector<T> &v) {
  sort(v.begin(), v.end());
  v.erase(unique(v.begin(), v.end()), v.end());
}

long long modp(long long a, long long p) {
  long long res = a % p;
  if (res < 0)
    res += p;
  return res;
}

void Main() {
  ll N, D, X;
  cin >> N >> X >> D;

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

  int flag = D > 0 ? 1 : -1;
  D = abs(D);

  VL mod{0};
  REP(i, N) {
    ll m = modp(mod.back() + X, D);
    mod.push_back(m);
  }
  remove_duplication(mod);
  map<ll, int> mp;
  REP(i, mod.size()) mp[mod[i]] = i;

  vector<vector<P>> v(mod.size());
  v[0].emplace_back(0, 0);
  ll m = 0;
  for (ll i = 1; i < N + 1; i++) {
    m = modp(m + X, D);
    int k = mp[m];
    ll mn = i * X / D + flag * i * (i - 1) / 2;
    ll mx = i * X / D + flag * (N * (N - 1) / 2 - (N - i) * (N - i - 1) / 2);
    if (flag < 0)
      swap(mn, mx);
    v[k].emplace_back(mn, mx);
  }

  ll ans = 0;
  REP(i, mp.size()) {
    SORT(v[i]);
    ll e = -LINF;
    REP(j, v[i].size()) {
      if (e < v[i][j].first)
        ans += v[i][j].second - v[i][j].first + 1;
      else if (e < v[i][j].second)
        ans += v[i][j].second - e;
      chmax(e, v[i][j].second);
    }
  }

  cout << ans << en;
  return;
}

int main(void) {
  cin.tie(0);
  ios_base::sync_with_stdio(0);
  cout << fixed << setprecision(15);
  int t = 1; // cin>>t;
  REP(_, t) { Main(); }
  return 0;
}