Problem p03356 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
template <class T> using vt = vector<T>;
template <class T> using vvt = vector<vt<T>>;
template <class T> using ttt = tuple<T, T>;
using tii = tuple<int, int>;
using vi = vector<int>;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define pb push_back
#define mt make_tuple
#define ALL(a) (a).begin(), (a).end()
#define FST first
#define SEC second
#define DEB cerr << "!" << endl
#define SHOW(a, b) cerr << (a) << " " << (b) << endl
#define DIV int(1e9 + 7)
const int INF = (INT_MAX / 2);
const ll LLINF = (LLONG_MAX / 2);
const double eps = 1e-8;
// const double PI = M_PI;
inline ll pow(ll x, ll n, ll m) {
  ll r = 1;
  while (n > 0) {
    if ((n & 1) == 1)
      r = r * x % m;
    x = x * x % m;
    n >>= 1;
  }
  return r % m;
}
inline ll lcm(ll d1, ll d2) { return d1 / __gcd(d1, d2) * d2; }

/*
願はくは花の下にて春死なんそのきさらぎの望月のころ（西行 1118-1190）
--This is old Japanese poem expressing the desire to die under tree on the full
moon night.(written by Saigyo) In my research, there is possibility that the
tree may be seg-tree.
*/

/*Coding Space*/
class UF {
public:
  vi value;
  vi over; // When over < 0, 'over' keep number of node(using negative number).
           // -1 means this tree is a node.
  int root(int index) {
    int t = index;
    stack<int> stack_i;
    while (over[t] >= 0) {
      stack_i.push(t);
      t = over[t];
    }
    while (!stack_i.empty()) {
      int i = stack_i.top();
      stack_i.pop();
      over[i] = t;
    } // reconnect
    return t;
  }
  void merge(int a, int b) {
    if (this->root(a) == this->root(b))
      return;
    over[root(a)] += over[root(b)]; // over have a number of nodes.
    over[root(b)] = root(a);
  }
  int NumNode(int a) { return -over[root(a)]; }
  UF(int Nodes) {
    value.resize(Nodes);
    over.resize(Nodes);
    rep(i, Nodes) over[i] = -1;
  }
};
UF uf(10001);
int main() {
  int n, m;
  cin >> n >> m;
  vi p(n + 1);
  rep(i, n) cin >> p[i + 1];
  rep(i, m) {
    int x, y;
    cin >> x >> y;
    uf.merge(x, y);
  }
  int ans = 0;
  rep(i, n) {
    if (uf.root(i + 1) == uf.root(p[i + 1]))
      ans++;
  }
  cout << ans << endl;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
template <class T> using vt = vector<T>;
template <class T> using vvt = vector<vt<T>>;
template <class T> using ttt = tuple<T, T>;
using tii = tuple<int, int>;
using vi = vector<int>;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define pb push_back
#define mt make_tuple
#define ALL(a) (a).begin(), (a).end()
#define FST first
#define SEC second
#define DEB cerr << "!" << endl
#define SHOW(a, b) cerr << (a) << " " << (b) << endl
#define DIV int(1e9 + 7)
const int INF = (INT_MAX / 2);
const ll LLINF = (LLONG_MAX / 2);
const double eps = 1e-8;
// const double PI = M_PI;
inline ll pow(ll x, ll n, ll m) {
  ll r = 1;
  while (n > 0) {
    if ((n & 1) == 1)
      r = r * x % m;
    x = x * x % m;
    n >>= 1;
  }
  return r % m;
}
inline ll lcm(ll d1, ll d2) { return d1 / __gcd(d1, d2) * d2; }

/*
願はくは花の下にて春死なんそのきさらぎの望月のころ（西行 1118-1190）
--This is old Japanese poem expressing the desire to die under tree on the full
moon night.(written by Saigyo) In my research, there is possibility that the
tree may be seg-tree.
*/

/*Coding Space*/
class UF {
public:
  vi value;
  vi over; // When over < 0, 'over' keep number of node(using negative number).
           // -1 means this tree is a node.
  int root(int index) {
    int t = index;
    stack<int> stack_i;
    while (over[t] >= 0) {
      stack_i.push(t);
      t = over[t];
    }
    while (!stack_i.empty()) {
      int i = stack_i.top();
      stack_i.pop();
      over[i] = t;
    } // reconnect
    return t;
  }
  void merge(int a, int b) {
    if (this->root(a) == this->root(b))
      return;
    over[root(a)] += over[root(b)]; // over have a number of nodes.
    over[root(b)] = root(a);
  }
  int NumNode(int a) { return -over[root(a)]; }
  UF(int Nodes) {
    value.resize(Nodes);
    over.resize(Nodes);
    rep(i, Nodes) over[i] = -1;
  }
};
UF uf(100001);
int main() {
  int n, m;
  cin >> n >> m;
  vi p(n + 1);
  rep(i, n) cin >> p[i + 1];
  rep(i, m) {
    int x, y;
    cin >> x >> y;
    uf.merge(x, y);
  }
  int ans = 0;
  rep(i, n) {
    if (uf.root(i + 1) == uf.root(p[i + 1]))
      ans++;
  }
  cout << ans << endl;
}