Language: C++

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

class UnionFind {
private:
  int N;
  vector<int> PS;

public:
  UnionFind() : N(0) {}
  UnionFind(int n) : N(n), PS(n, -1) {}
  int unite(int a, int b) {
    int x = root(a), y = root(b);
    if (x == y) return x;
    if (-PS.at(x) < -PS.at(y)) swap(x, y);
    PS.at(x) += PS.at(y);
    PS.at(y) = x;
    return x;
  }
  bool same(int a, int b) { return root(a) == root(b); }
  int root(int a) {
    if (PS.at(a) < 0) return a;
    return PS.at(a) = root(PS.at(a));
  }
  int size(int a) { return -PS.at(root(a)); }
  vector<vector<int>> groups() {
    vector<int> B(N), S(N);
    for (int i = 0; i < N; i++) {
      B.at(i) = root(i);
      S.at(B.at(i))++;
    }
    vector<vector<int>> ret(N);
    for (int i = 0; i < N; i++) {
      ret.at(i).reserve(S.at(i));
    }
    for (int i = 0; i < N; i++) {
      ret.at(B.at(i)).push_back(i);
    }
    auto f = [&](const vector<int> &v) { return v.empty(); };
    ret.erase(remove_if(ret.begin(), ret.end(), f), ret.end());
    return ret;
  }
};

int main() {
  int N, M;
  cin >> N >> M;
  UnionFind UF(N);
  for (int i = 0; i < M; i++) {
    int A, B;
    cin >> A >> B, A--, B--;
    UF.unite(A, B);
  }
  cout << UF.groups() - 1 << "\n";
}

Compiler output:
a.cc: In function 'int main()':
a.cc:54:23: error: no match for 'operator-' (operand types are 'std::vector<std::vector<int> >' and 'int')
   54 |   cout << UF.groups() - 1 << "\n";
      |           ~~~~~~~~~~~ ^ ~
      |                    |    |
      |                    |    int
      |                    std::vector<std::vector<int> >
In file included from /usr/include/c++/14/bits/stl_algobase.h:67,
                 from /usr/include/c++/14/algorithm:60,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_iterator.h:618:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr decltype ((__y.base() - __x.base())) std::operator-(const reverse_iterator<_Iterator>&, const reverse_iterator<_IteratorR>&)'
  618 |     operator-(const reverse_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:618:5: note:   template argument deduction/substitution failed:
a.cc:54:25: note:   'std::vector<std::vector<int> >' is not derived from 'const std::reverse_iterator<_Iterator>'
   54 |   cout << UF.groups() - 1 << "\n";
      |                         ^
/usr/include/c++/14/bits/stl_iterator.h:1790:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr decltype ((__x.base() - __y.base())) std::operator-(const move_iterator<_IteratorL>&, const move_iterator<_IteratorR>&)'
 1790 |     operator-(const move_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1790:5: note:   template argument deduction/substitution failed:
a.cc:54:25: note:   'std::vector<std::vector<int> >' is not derived from 'const std::move_iterator<_IteratorL>'
   54 |   cout << UF.groups() - 1 << "\n";
      |                         ^
In file included from /usr/include/c++/14/ccomplex:39,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:127:
/usr/include/c++/14/complex:370:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::operator-(const complex<_Tp>&, const complex<_Tp>&)'
  370 |     operator-(const complex<_Tp>& __x, const complex<_Tp>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/complex:370:5: note:   template argument deduction/substitution failed:
a.cc:54:25: note:   'std::vector<std::vector<int> >' is not derived from 'const std::complex<_Tp>'
   54 |   cout << UF.groups() - 1 << "\n";
      |                         ^
/usr/include/c++/14/complex:379:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::operator-(const complex<_Tp>&, const _Tp&)'
  379 |     operator-(const complex<_Tp>& __x, const _Tp& __y)
      |     ^~~~~~~~
/usr/include/c++/14/complex:379:5: note:   template argument deduction/substitution failed:
a.cc:54:25: note:   'std::vector<std::vector<int> >' is not derived from 'const std::complex<_Tp>'
   54 |   cout << UF.groups() - 1 << "\n";
      |                         ^
/usr/include/c++/14/complex:388:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::operator-(const _Tp&, const complex<_Tp>&)'
  388 |     operator-(const _Tp& __x, const complex<_Tp>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/complex:388:5: note:   template argument deduction/substitution failed:
a.cc:54:25: note:   mismatched types 'const std::complex<_Tp>' and 'int'
   54 |   cout << UF.groups() - 1 << "\n";
      |                         ^
/usr/include/c++/14/complex:465:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::operator-(const complex<_Tp>&)'
  465 |     operator-(const complex<_Tp>& __x)
      |     ^~~~~~~~
/usr/include/c++/14/complex:465:5: note:   candidate expects 1 argument, 2 provided
In file included from /usr/include/c++/14/valarray:605,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:166:
/usr/include/c++/14/bits/valarray_after.h:406:5: note: candidate: 'template<class _Dom1, class _Dom2> std::_Expr<std::__detail::_BinClos<std::__minus, std::_Expr, std::_Expr, _Dom1, _Dom2>, typename std::__fun<std::__minus, typename _Dom1::value_type>::result_type> std::operator-(const _Expr<_Dom1, typename _Dom1::value_type>&, const _Expr<_Dom2, typename _Dom2::value_type>&)'
  406 |     _DEFINE_EXPR_BINARY_OPERATOR(-, struct std::__minus)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:406:5: note:   template argument deduction/substitution failed:
a.cc:54:25: note:   'std::vector<std::vector<int> >' is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
   54 |   cout << UF.groups() - 1 << "\n";
      |                         ^
/usr/include/c++/14/bits/valarray_after.h:406:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__minus, std::_Expr, std::_Constant, _Dom, typename _Dom::value_type>, typename std::__fun<std::__minus, typename _Dom1::value_type>::result_type> std::operator-(const _Expr<_Dom1, typename _Dom1::value_type>&, const typename _Dom::value_type&)'
  406 |     _DEFINE_EXPR_BINARY_OPERATOR(-, struct std::__minus)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:406:5: note:   template argument deduction/substitution failed:
a.cc:54:25: note:   'std::vector<std::vector<int> >' is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
   54 |   cout << UF.groups() - 1 << "\n";
      |                         ^
/usr/include/c++/14/bits/valarray_after.h:406:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__minus, std::_Constant, std::_Expr, typename _Dom::value_type, _Dom>, typename std::__fun<std::__minus, typename _Dom1::value_type>::result_type> std::operator-(const typename _Dom::value_type&, const _Expr<_Dom1, typename _Dom1::value_type>&)'
  406 |     _DEFINE_EXPR_BINARY_OPERATOR(-, struct std::__minus)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:406:5: note:   template argument deduction/substitution failed:
a.cc:54:25: note:   mismatched types 'const std::_Expr<_Dom1, typename _Dom1::value_type>' and 'int'
   54 |   cout << UF.groups() - 1 << "\n";
      |                         ^
/usr/include/c++/14/bits/valarray_after.h:406:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__minus, std::_Expr, std::_ValArray, _Dom, typename _Dom::value_type>, typename std::__fun<std::__minus, typename _Dom1::value_type>::result_type> std::operator-(const _Expr<_Dom1, typename _Dom1::value_type>&, const valarray<typename _Dom::value_type>&)'
  406 |     _DEFINE_EXPR_BINARY_OPERATOR(-, struct std::__minus)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:406:5: note:   template argument deduction/substitution failed:
a.cc:54:25: note:   'std::vector<std::vector<int> >' is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
   54 |   cout << UF.groups() - 1 << "\n";
      |                         ^
/usr/include/c++/14/bits/valarray_after.h:406:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__minus, std::_ValArray, std::_Expr, typename _Dom::value_type, _Dom>, typename std::__fun<std::__minus, typename _Dom1::value_type>::result_type> std::operator-(const valarray<typename _Dom::value_type>&, const _Expr<_Dom1, typename _Dom1::value_type>&)'
  406 |     _DEFINE_EXPR_BINARY_OPERATOR(-, struct std::__minus)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:406:5: note:   template argument deduction/substitution failed:
a.cc:54:25: note:   mismatched types 'const std::_Expr<_Dom1, typename _Dom1::value_type>' and 'int'
   54 |   cout << UF.groups() - 1 << "\n";
      |                         ^
/usr/include/c++/14/valarray:1197:1: note: candidate: 'template<class _Tp> std::_Expr<std::__detail::_BinClos<std::__minus, std::_ValArray, std::_ValArray, _Tp, _Tp>, typename std::__fun<std::__minus, _Tp>::result_type> std::operator-(const valarray<_Tp>&, const valarray<_Tp>&)'
 1197 | _DEFINE_BINARY_OPERATOR(-, __minus)
      | ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/valarray:1197:1: note:   template argument deduction/substitution failed:
a.cc:54:25: note:   'std::vector<std::vector<int> >' is not derived from 'const std::valarray<_Tp>'
   54 |   cout << UF.groups() - 1 << "\n";
      |                         ^
/usr/include/c++/14/valarray:1197:1: note: candidate: 'template<class _Tp> std::_Expr<std::__detail::_BinClos<std::__minus, std::_ValArray, std::_Constant, _Tp, _Tp>, typename std::__fun<std::__minus, _Tp>::result_type> std::operator-(const valarray<_Tp>&, const typename valarray<_Tp>::value_type&)'
 1197 | _DEFINE_BINARY_OPERATOR(-, __minus)
      | ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/valarray:1197:1: note:   template argument deduction/substitution failed:
a.cc:54:25: note:   'std::vector<std::vector<int> >' is not derived from 'const std::valarray<_Tp>'
   54 |   cout << UF.groups() - 1 << "\n";
      |                         ^
/usr/include/c++/14/valarray:1197:1: note: candidate: 'template<class _Tp> std::_Expr<std::__detail::_BinClos<std::__minus, std::_Constant, std::_ValArray, _Tp, _Tp>, typename std::__fun<std::__minus, _Tp>::result_type> std::operator-(const typename valarray<_Tp>::value_type&, const valarray<_Tp>&)'
 1197 | _DEFINE_BINARY_OPERATOR(-, __minus)
      | ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/valarray:1197:1: note:   template argument deduction/substitution failed:
a.cc:54:25: note:   mismatched types 'const std::valarray<_Tp>' and 'int'
   54 |   cout << UF.groups() - 1 << "\n";
      |                         ^