Language: C++

#include <iostream>
using namespace std;

int par[50001];
int rank[50001];

void init(int n)
{
  for(int i = 0; i < n; i++){
    par[i] =i;
    rank[i] = 0;
  }
}

int find(int x)
{
  if(par[x] == x){
    return x;
  } else {
    return par[x] = find(par[x]);
  }
}

void unite(int x, int y)
{
  x = find(x);
  y = find(y);
  if(x == y) return;

  if(rank[x] < rank[y]){
    par[x] = y;
  } else {
    par[y] = x;
    if(rank[x] == rank[y]) rank[x]++;
  }
}

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

int main()
{
  int n, m;
  int p[50001];
  int x, y;
  int ans = 0;

  cin >> n >> m;

  init(n);
  for(int i = 1; i <= n; i++)
    cin >> p[i];
  for(int i = 0; i < m; i++){
    cin >> x >> y;
    unite(x, y);
  }

  for(int i = 1; i <= n; i++){
    if(same(i, p[i])) ans++;
  }

  cout << ans << endl;

  return 0;
}

Compiler output:
a.cc: In function 'void init(int)':
a.cc:11:5: error: reference to 'rank' is ambiguous
   11 |     rank[i] = 0;
      |     ^~~~
In file included from /usr/include/c++/14/bits/move.h:37,
                 from /usr/include/c++/14/bits/exception_ptr.h:41,
                 from /usr/include/c++/14/exception:166,
                 from /usr/include/c++/14/ios:41,
                 from /usr/include/c++/14/ostream:40,
                 from /usr/include/c++/14/iostream:41,
                 from a.cc:1:
/usr/include/c++/14/type_traits:1437:12: note: candidates are: 'template<class> struct std::rank'
 1437 |     struct rank
      |            ^~~~
a.cc:5:5: note:                 'int rank [50001]'
    5 | int rank[50001];
      |     ^~~~
a.cc: In function 'void unite(int, int)':
a.cc:30:6: error: reference to 'rank' is ambiguous
   30 |   if(rank[x] < rank[y]){
      |      ^~~~
/usr/include/c++/14/type_traits:1437:12: note: candidates are: 'template<class> struct std::rank'
 1437 |     struct rank
      |            ^~~~
a.cc:5:5: note:                 'int rank [50001]'
    5 | int rank[50001];
      |     ^~~~
a.cc:30:16: error: reference to 'rank' is ambiguous
   30 |   if(rank[x] < rank[y]){
      |                ^~~~
/usr/include/c++/14/type_traits:1437:12: note: candidates are: 'template<class> struct std::rank'
 1437 |     struct rank
      |            ^~~~
a.cc:5:5: note:                 'int rank [50001]'
    5 | int rank[50001];
      |     ^~~~
a.cc:34:8: error: reference to 'rank' is ambiguous
   34 |     if(rank[x] == rank[y]) rank[x]++;
      |        ^~~~
/usr/include/c++/14/type_traits:1437:12: note: candidates are: 'template<class> struct std::rank'
 1437 |     struct rank
      |            ^~~~
a.cc:5:5: note:                 'int rank [50001]'
    5 | int rank[50001];
      |     ^~~~
a.cc:34:19: error: reference to 'rank' is ambiguous
   34 |     if(rank[x] == rank[y]) rank[x]++;
      |                   ^~~~
/usr/include/c++/14/type_traits:1437:12: note: candidates are: 'template<class> struct std::rank'
 1437 |     struct rank
      |            ^~~~
a.cc:5:5: note:                 'int rank [50001]'
    5 | int rank[50001];
      |     ^~~~
a.cc:34:28: error: reference to 'rank' is ambiguous
   34 |     if(rank[x] == rank[y]) rank[x]++;
      |                            ^~~~
/usr/include/c++/14/type_traits:1437:12: note: candidates are: 'template<class> struct std::rank'
 1437 |     struct rank
      |            ^~~~
a.cc:5:5: note:                 'int rank [50001]'
    5 | int rank[50001];
      |     ^~~~