Problem p02998 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<ll, ll> P;
typedef pair<int, int> Pi;
#define rep(i, n) for (ll i = 0; i < n; i++)
#define FOR(i, a, b) for (ll i = a; i < b; i++)
#define fi first
#define se second
#define endl "\n"

template <typename T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> ostream &operator<<(ostream &s, const complex<T> &d) {
  return s << "(" << d.real() << ", " << d.imag() << ")";
}
template <typename T1, typename T2>
ostream &operator<<(ostream &s, const pair<T1, T2> &d) {
  return s << "(" << d.first << ", " << d.second << ")";
}
template <typename T> ostream &operator<<(ostream &s, const vector<T> &d) {
  int len = d.size();
  rep(i, len) {
    s << d[i];
    if (i < len - 1)
      s << " ";
  }
  return s;
}
template <typename T>
ostream &operator<<(ostream &s, const vector<vector<T>> &d) {
  int len = d.size();
  rep(i, len) { s << d[i] << endl; }
  return s;
}
template <typename T> ostream &operator<<(ostream &s, const set<T> &v) {
  s << "{ ";
  for (auto itr = v.begin(); itr != v.end(); ++itr) {
    if (itr != v.begin()) {
      s << ", ";
    }
    s << (*itr);
  }
  s << " }";
  return s;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &s, const map<T1, T2> &m) {
  s << "{" << endl;
  for (auto itr = m.begin(); itr != m.end(); ++itr) {
    s << " " << (*itr).first << " : " << (*itr).second << endl;
  }
  s << "}" << endl;
  return s;
}

const ll mod = 1e9 + 7;
const ll inf = 1e17;
const int INF = 1e9;
const double PI = acos(-1);
const double EPS = 1e-10;

int n, offset = 100010;
vector<vector<int>> g(300010);
vector<bool> used(300010, false);
vector<ll> cnt(2);

void dfs(int v) {
  if (used[v])
    return;
  used[v] = true;
  if (v >= offset)
    cnt[1]++;
  else
    cnt[0]++;
  for (auto itr : g[v]) {
    dfs(itr);
  }
}

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cin >> n;
  vector<int> ten;
  rep(i, n) {
    int x, y;
    cin >> x >> y;
    y += offset;
    ten.push_back(x);
    ten.push_back(y);
    g[x].push_back(y);
    g[y].push_back(x);
  }
  ll ans = 0;
  for (auto itr : ten) {
    if (used[ten[itr]])
      continue;
    rep(j, 2) cnt[j] = 0;
    dfs(itr);
    ans += cnt[0] * cnt[1];
  }
  ans -= n;
  cout << ans << endl;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<ll, ll> P;
typedef pair<int, int> Pi;
#define rep(i, n) for (ll i = 0; i < n; i++)
#define FOR(i, a, b) for (ll i = a; i < b; i++)
#define fi first
#define se second
#define endl "\n"

template <typename T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> ostream &operator<<(ostream &s, const complex<T> &d) {
  return s << "(" << d.real() << ", " << d.imag() << ")";
}
template <typename T1, typename T2>
ostream &operator<<(ostream &s, const pair<T1, T2> &d) {
  return s << "(" << d.first << ", " << d.second << ")";
}
template <typename T> ostream &operator<<(ostream &s, const vector<T> &d) {
  int len = d.size();
  rep(i, len) {
    s << d[i];
    if (i < len - 1)
      s << " ";
  }
  return s;
}
template <typename T>
ostream &operator<<(ostream &s, const vector<vector<T>> &d) {
  int len = d.size();
  rep(i, len) { s << d[i] << endl; }
  return s;
}
template <typename T> ostream &operator<<(ostream &s, const set<T> &v) {
  s << "{ ";
  for (auto itr = v.begin(); itr != v.end(); ++itr) {
    if (itr != v.begin()) {
      s << ", ";
    }
    s << (*itr);
  }
  s << " }";
  return s;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &s, const map<T1, T2> &m) {
  s << "{" << endl;
  for (auto itr = m.begin(); itr != m.end(); ++itr) {
    s << " " << (*itr).first << " : " << (*itr).second << endl;
  }
  s << "}" << endl;
  return s;
}

const ll mod = 1e9 + 7;
const ll inf = 1e17;
const int INF = 1e9;
const double PI = acos(-1);
const double EPS = 1e-10;

int n, offset = 100010;
vector<vector<int>> g(300010);
vector<bool> used(300010, false);
vector<ll> cnt(2);

void dfs(int v) {
  if (used[v])
    return;
  used[v] = true;
  if (v >= offset)
    cnt[1]++;
  else
    cnt[0]++;
  for (auto itr : g[v]) {
    dfs(itr);
  }
}

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cin >> n;
  vector<int> ten;
  rep(i, n) {
    int x, y;
    cin >> x >> y;
    y += offset;
    ten.push_back(x);
    ten.push_back(y);
    g[x].push_back(y);
    g[y].push_back(x);
  }
  ll ans = 0;
  for (auto itr : ten) {
    if (used[itr])
      continue;
    rep(j, 2) cnt[j] = 0;
    dfs(itr);
    ans += cnt[0] * cnt[1];
  }
  ans -= n;
  cout << ans << endl;
}