Problem p02536 (C++)

Buggy submission (status: Runtime Error):
//@formatter:off
#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < int(n); ++i)
#define rrep(i, n) for (int i = int(n) - 1; i >= 0; i--)
#define rep2(i, s, n) for (int i = int(s); i < int(n); ++i)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define pb push_back
#define eb emplace_back
#define vi vector<int>
#define vvi vector<vector<int>>
#define vl vector<ll>
#define vvl vector<vector<ll>>
#define vd vector<double>
#define vvd vector<vector<double>>
#define vs vector<string>
#define vc vector<char>
#define vvc vector<vector<char>>
#define vb vector<bool>
#define vvb vector<vector<bool>>
#define vp vector<P>
#define vvp vector<vector<P>>
using namespace std;
using ll = long long;
using P = pair<int, int>;
using LP = pair<ll, ll>;
template <class S, class T> istream &operator>>(istream &is, pair<S, T> &p) {
  return is >> p.first >> p.second;
}
template <class S, class T>
ostream &operator<<(ostream &os, const pair<S, T> &p) {
  return os << '{' << p.first << "," << p.second << '}';
}
template <class T> istream &operator>>(istream &is, vector<T> &v) {
  for (T &t : v) {
    is >> t;
  }
  return is;
}
template <class T> ostream &operator<<(ostream &os, const vector<T> &v) {
  os << '[';
  rep(i, v.size()) os << v[i] << (i == int(v.size() - 1) ? "" : ",");
  return os << ']';
}
void Yes(bool b) { cout << (b ? "Yes" : "No") << '\n'; }
void YES(bool b) { cout << (b ? "YES" : "NO") << '\n'; }
template <class T> bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
const int inf = 1001001001;
const ll linf = 1001001001001001001;
//@formatter:on

class unionfind {
  int n;
  vector<int> par, rank;

public:
  unionfind(int n) : n(n), par(n, -1), rank(n, 0) {}

  int root(int x) {
    if (par[x] < 0)
      return x;
    else
      return par[x] = root(par[x]);
  }

  bool same(int x, int y) { return root(x) == root(y); };

  bool merge(int x, int y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (rank[x] < rank[y])
      swap(x, y);
    if (rank[x] == rank[y])
      rank[x]++;
    par[x] += par[y];
    par[y] = x;
    return true;
  }

  int size(int x) { return -par[root(x)]; };

  bool connected() {
    rep(i, n - 1) if (!same(i, i + 1)) return false;
    return true;
  }
};

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  int n, m;
  cin >> n >> m;
  unionfind uf(n);
  rep(i, m) {
    int a, b;
    cin >> a >> b;
    uf.merge(a, b);
  }
  int ans = -1;
  rep(i, n) {
    if (uf.root(i) == i)
      ans++;
  }
  cout << ans << endl;
}

Fixed submission:
//@formatter:off
#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < int(n); ++i)
#define rrep(i, n) for (int i = int(n) - 1; i >= 0; i--)
#define rep2(i, s, n) for (int i = int(s); i < int(n); ++i)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define pb push_back
#define eb emplace_back
#define vi vector<int>
#define vvi vector<vector<int>>
#define vl vector<ll>
#define vvl vector<vector<ll>>
#define vd vector<double>
#define vvd vector<vector<double>>
#define vs vector<string>
#define vc vector<char>
#define vvc vector<vector<char>>
#define vb vector<bool>
#define vvb vector<vector<bool>>
#define vp vector<P>
#define vvp vector<vector<P>>
using namespace std;
using ll = long long;
using P = pair<int, int>;
using LP = pair<ll, ll>;
template <class S, class T> istream &operator>>(istream &is, pair<S, T> &p) {
  return is >> p.first >> p.second;
}
template <class S, class T>
ostream &operator<<(ostream &os, const pair<S, T> &p) {
  return os << '{' << p.first << "," << p.second << '}';
}
template <class T> istream &operator>>(istream &is, vector<T> &v) {
  for (T &t : v) {
    is >> t;
  }
  return is;
}
template <class T> ostream &operator<<(ostream &os, const vector<T> &v) {
  os << '[';
  rep(i, v.size()) os << v[i] << (i == int(v.size() - 1) ? "" : ",");
  return os << ']';
}
void Yes(bool b) { cout << (b ? "Yes" : "No") << '\n'; }
void YES(bool b) { cout << (b ? "YES" : "NO") << '\n'; }
template <class T> bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
const int inf = 1001001001;
const ll linf = 1001001001001001001;
//@formatter:on

class unionfind {
  int n;
  vector<int> par, rank;

public:
  unionfind(int n) : n(n), par(n, -1), rank(n, 0) {}

  int root(int x) {
    if (par[x] < 0)
      return x;
    else
      return par[x] = root(par[x]);
  }

  bool same(int x, int y) { return root(x) == root(y); };

  bool merge(int x, int y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (rank[x] < rank[y])
      swap(x, y);
    if (rank[x] == rank[y])
      rank[x]++;
    par[x] += par[y];
    par[y] = x;
    return true;
  }

  int size(int x) { return -par[root(x)]; };

  bool connected() {
    rep(i, n - 1) if (!same(i, i + 1)) return false;
    return true;
  }
};

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  int n, m;
  cin >> n >> m;
  unionfind uf(n);
  rep(i, m) {
    int a, b;
    cin >> a >> b;
    a--;
    b--;
    uf.merge(a, b);
  }
  int ans = -1;
  rep(i, n) {
    if (uf.root(i) == i)
      ans++;
  }
  cout << ans << endl;
}