Problem p03108 (C++)

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

// #define int long long
#define rep(i, n) for (long long i = (long long)(0); i < (long long)(n); ++i)
#define reps(i, n) for (long long i = (long long)(1); i <= (long long)(n); ++i)
#define rrep(i, n) for (long long i = ((long long)(n)-1); i >= 0; i--)
#define rreps(i, n) for (long long i = ((long long)(n)); i > 0; i--)
#define irep(i, m, n)                                                          \
  for (long long i = (long long)(m); i < (long long)(n); ++i)
#define ireps(i, m, n)                                                         \
  for (long long i = (long long)(m); i <= (long long)(n); ++i)
#define SORT(v, n) sort(v, v + n);
#define REVERSE(v, n) reverse(v, v + n);
#define vsort(v) sort(v.begin(), v.end());
#define all(v) v.begin(), v.end()
#define mp(n, m) make_pair(n, m);
#define cout(d) cout << d << endl;
#define coutd(d) cout << std::setprecision(10) << d << endl;
#define cinline(n) getline(cin, n);
#define replace_all(s, b, a) replace(s.begin(), s.end(), b, a);
#define PI (acos(-1))
#define FILL(v, n, x) fill(v, v + n, x);
#define sz(x) long long(x.size())

using ll = long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vll = vector<ll>;
using vvll = vector<vll>;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using vs = vector<string>;
using vpll = vector<pair<ll, ll>>;
using vtp = vector<tuple<ll, ll, ll>>;
using vb = vector<bool>;

template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}

const ll INF = 1e15;
const ll MOD = 1e9 + 7;
const ll LINF = 1e18;

class UnionFind {
private:
  vector<ll> Parent;

public:
  UnionFind(ll N) { Parent = vector<ll>(N, -1); }

  ll root(ll A) {
    if (Parent[A] < 0)
      return A;
    return Parent[A] = root(Parent[A]);
  }

  ll size(ll A) { return -Parent[root(A)]; }

  bool connect(ll A, ll B) {
    A = root(A);
    B = root(B);
    if (A == B) {
      return false;
    }
    if (size(A) < size(B))
      swap(A, B);
    Parent[A] += Parent[B];
    Parent[B] = A;
    return true;
  }

  bool isSame(ll A, ll B) { return root(A) == root(B); }
};

signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  ll n, m;
  cin >> n >> m;
  UnionFind uf(n);
  vll ans(m);
  ans[0] = n * (n - 1) / 2;
  vpll ab(m);
  rep(i, m) cin >> ab[i].first >> ab[i].second;
  reverse(all(ab));
  rep(i, m - 1) {
    ll a = ab[i].first, b = ab[i].second;
    if (!uf.isSame(a, b)) {
      ans[i + 1] = ans[i] - uf.size(a) * uf.size(b);
      uf.connect(a, b);
    } else {
      ans[i + 1] = ans[i];
    }
  }
  reverse(all(ans));
  rep(i, m) { cout << ans[i] << endl; }
}

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

// #define int long long
#define rep(i, n) for (long long i = (long long)(0); i < (long long)(n); ++i)
#define reps(i, n) for (long long i = (long long)(1); i <= (long long)(n); ++i)
#define rrep(i, n) for (long long i = ((long long)(n)-1); i >= 0; i--)
#define rreps(i, n) for (long long i = ((long long)(n)); i > 0; i--)
#define irep(i, m, n)                                                          \
  for (long long i = (long long)(m); i < (long long)(n); ++i)
#define ireps(i, m, n)                                                         \
  for (long long i = (long long)(m); i <= (long long)(n); ++i)
#define SORT(v, n) sort(v, v + n);
#define REVERSE(v, n) reverse(v, v + n);
#define vsort(v) sort(v.begin(), v.end());
#define all(v) v.begin(), v.end()
#define mp(n, m) make_pair(n, m);
#define cout(d) cout << d << endl;
#define coutd(d) cout << std::setprecision(10) << d << endl;
#define cinline(n) getline(cin, n);
#define replace_all(s, b, a) replace(s.begin(), s.end(), b, a);
#define PI (acos(-1))
#define FILL(v, n, x) fill(v, v + n, x);
#define sz(x) long long(x.size())

using ll = long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vll = vector<ll>;
using vvll = vector<vll>;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using vs = vector<string>;
using vpll = vector<pair<ll, ll>>;
using vtp = vector<tuple<ll, ll, ll>>;
using vb = vector<bool>;

template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}

const ll INF = 1e15;
const ll MOD = 1e9 + 7;
const ll LINF = 1e18;

class UnionFind {
private:
  vector<ll> Parent;

public:
  UnionFind(ll N) { Parent = vector<ll>(N, -1); }

  ll root(ll A) {
    if (Parent[A] < 0)
      return A;
    return Parent[A] = root(Parent[A]);
  }

  ll size(ll A) { return -Parent[root(A)]; }

  bool connect(ll A, ll B) {
    A = root(A);
    B = root(B);
    if (A == B) {
      return false;
    }
    if (size(A) < size(B))
      swap(A, B);
    Parent[A] += Parent[B];
    Parent[B] = A;
    return true;
  }

  bool isSame(ll A, ll B) { return root(A) == root(B); }
};

signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  ll n, m;
  cin >> n >> m;
  UnionFind uf(n);
  vll ans(m);
  ans[0] = n * (n - 1) / 2;
  vpll ab(m);
  rep(i, m) cin >> ab[i].first >> ab[i].second, ab[i].first--, ab[i].second--;
  reverse(all(ab));
  rep(i, m - 1) {
    ll a = ab[i].first, b = ab[i].second;
    if (!uf.isSame(a, b)) {
      ans[i + 1] = ans[i] - uf.size(a) * uf.size(b);
      uf.connect(a, b);
    } else {
      ans[i + 1] = ans[i];
    }
  }
  reverse(all(ans));
  rep(i, m) { cout << ans[i] << endl; }
}