Problem p02840 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bitset>
#include <ciso646>
#include <cmath>
#include <complex>
#include <cstdio>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <utility>
#include <vector>
using namespace std;
typedef long long ll;
typedef unsigned int ui;
const ll mod = 1000000007;
const ll INF = (ll)1000000007 * 1000000007;
typedef pair<int, int> P;
#define stop                                                                   \
  char nyaa;                                                                   \
  cin >> nyaa;
#define rep(i, n) for (int i = 0; i < n; i++)
#define per(i, n) for (int i = n - 1; i >= 0; i--)
#define Rep(i, sta, n) for (int i = sta; i < n; i++)
#define Per(i, sta, n) for (int i = n - 1; i >= sta; i--)
#define rep1(i, n) for (int i = 1; i <= n; i++)
#define per1(i, n) for (int i = n; i >= 1; i--)
#define Rep1(i, sta, n) for (int i = sta; i <= n; i++)
typedef long double ld;
typedef complex<ld> Point;
const ld eps = 1e-8;
const ld pi = acos(-1.0);
typedef pair<ll, ll> LP;

ll n, x, d;
unordered_map<ll, vector<LP>> D;

void solve() {
  cin >> n >> x >> d;
  if (d < 0) {
    x = (n - 1) * d + x;
    d *= -1;
  }
  for (ll i = 0; i <= n; i++) {
    D[i * x % d].push_back(LP(i * (i - 1) * d / 2 + i * x, 0));
    D[i * x % d].push_back(LP(i * (2 * n - i - 1) * d / 2 + i * x, 1));
  }
  ll ans = 0;
  for (auto L : D) {
    sort(L.second.begin(), L.second.end());
    int a = 0;
    ll fir;
    for (LP p : L.second) {
      // cout << p.first <<" " << p.second << endl;
      if (p.second == 0) {
        if (a == 0)
          fir = p.first;
        a += 1;
      } else {
        if (a == 1)
          ans += (p.first - fir) / d + 1;
        a -= 1;
      }
      // cout << ans << endl;
    }
  }
  cout << ans << endl;
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout << fixed << setprecision(50);
  solve();
}

Fixed submission:
#include <algorithm>
#include <bitset>
#include <ciso646>
#include <cmath>
#include <complex>
#include <cstdio>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <utility>
#include <vector>
using namespace std;
typedef long long ll;
typedef unsigned int ui;
const ll mod = 1000000007;
const ll INF = (ll)1000000007 * 1000000007;
typedef pair<int, int> P;
#define stop                                                                   \
  char nyaa;                                                                   \
  cin >> nyaa;
#define rep(i, n) for (int i = 0; i < n; i++)
#define per(i, n) for (int i = n - 1; i >= 0; i--)
#define Rep(i, sta, n) for (int i = sta; i < n; i++)
#define Per(i, sta, n) for (int i = n - 1; i >= sta; i--)
#define rep1(i, n) for (int i = 1; i <= n; i++)
#define per1(i, n) for (int i = n; i >= 1; i--)
#define Rep1(i, sta, n) for (int i = sta; i <= n; i++)
typedef long double ld;
typedef complex<ld> Point;
const ld eps = 1e-8;
const ld pi = acos(-1.0);
typedef pair<ll, ll> LP;

ll n, x, d;
unordered_map<ll, vector<LP>> D;

void solve() {
  cin >> n >> x >> d;
  if (d == 0) {
    if (x == 0) {
      cout << 1 << endl;
      return;
    }
    cout << n + 1 << endl;
    return;
  }
  if (d < 0) {
    x = (n - 1) * d + x;
    d *= -1;
  }
  for (ll i = 0; i <= n; i++) {
    D[i * x % d].push_back(LP(i * (i - 1) * d / 2 + i * x, 0));
    D[i * x % d].push_back(LP(i * (2 * n - i - 1) * d / 2 + i * x, 1));
  }
  ll ans = 0;
  for (auto L : D) {
    sort(L.second.begin(), L.second.end());
    int a = 0;
    ll fir;
    for (LP p : L.second) {
      // cout << p.first <<" " << p.second << endl;
      if (p.second == 0) {
        if (a == 0)
          fir = p.first;
        a += 1;
      } else {
        if (a == 1)
          ans += (p.first - fir) / d + 1;
        a -= 1;
      }
      // cout << ans << endl;
    }
  }
  cout << ans << endl;
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout << fixed << setprecision(50);
  solve();
}