Problem p03354 (C++)

Buggy submission (status: Runtime Error):
#pragma GCC optimize("O3")
#pragma GCC target("sse4")
#pragma GCC target("avx2,tune=native")
#include <bits/stdc++.h>
// #include<ext/pb_ds/tree_policy.hpp>
// #include<ext/pb_ds/assoc_container.hpp>
#define pb push_back
#define __V vector
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define oit ostream_iterator
#define mod 1000000007ll
using namespace std;
void doin() {
  cin.tie();
  cout.tie();
  ios::sync_with_stdio(0);
#ifndef ONLINE_JUDGE
//	freopen("input.txt", "r", stdin);
//	freopen("output.txt", "w", stdout);
#endif
}
template <typename X, typename Y>
istream &operator>>(istream &in, pair<X, Y> &a) {
  in >> a.first >> a.second;
  return in;
}
template <typename T> void getv(T &i) { cin >> i; }
template <typename T, typename... Ns> void getv(vector<T> &v, int n, Ns... ns) {
  v.resize(n);
  for (auto &i : v)
    getv(i, ns...);
}
template <typename T> void getv1(T &i) { cin >> i; }
template <typename T, typename... Ns>
void getv1(vector<T> &v, int n, Ns... ns) {
  v.resize(n + 1);
  for (int i = 1; i <= n; i++)
    getv1(v[i], ns...);
}
#ifdef _WIN32
#define getchar_unlocked() _getchar_nolock()
#define _CRT_DISABLE_PERFCRIT_LOCKS
#endif
inline void getch(char &x) {
  while (x = getchar_unlocked(), x < 33) {
    ;
  }
}
inline void getstr(string &str) {
  str.clear();
  char cur;
  while (cur = getchar_unlocked(), cur < 33) {
    ;
  }
  while (cur > 32) {
    str += cur;
    cur = getchar_unlocked();
  }
}
template <typename T> inline bool sc(T &num) {
  bool neg = 0;
  int c;
  num = 0;
  while (c = getchar_unlocked(), c < 33) {
    if (c == EOF)
      return false;
  }
  if (c == '-') {
    neg = 1;
    c = getchar_unlocked();
  }
  for (; c > 47; c = getchar_unlocked())
    num = num * 10 + c - 48;
  if (neg)
    num *= -1;
  return true;
}
template <typename a, typename b> void minq(a &x, b y) {
  if (x > y)
    x = y;
}
template <typename a, typename b> void maxq(a &x, b y) {
  if (x < y)
    x = y;
}
typedef unsigned long long ull;
typedef long long ll;
typedef float ld;
typedef ll _I;
typedef pair<_I, _I> pi;
typedef pair<ld, ld> pd;
typedef map<_I, _I> mii;
typedef __V<_I> vi;
typedef __V<char> vc;
typedef __V<ld> vd;
typedef __V<vd> vvd;
typedef __V<pi> vpi;
typedef __V<__V<_I>> vvi;
typedef __V<__V<char>> vvc;
typedef __V<__V<pi>> vvpi;
using AntonTsypko = void;
using arsijo = AntonTsypko;
using god = arsijo;
uniform_real_distribution<double> double_dist(0, 1);
// WHEN DOING MODULAR SUBTRACTION ALWAYS **ALWAYS** ADD MOD
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
struct dsu {
  vi p, r;
  void init(int n) {
    r.assign(n + 1, 0);
    p.resize(n + 1);
    for (int i = 0; i <= n; i++)
      p[i] = i;
  }
  int par(int i) { return p[i] == i ? i : p[i] = par(p[i]); }
  bool same(int i, int j) { return par(i) == par(j); }
  void unite(int i, int j) {
    i = par(i), j = par(j);
    if (i == j)
      return;
    if (r[i] < r[j])
      swap(i, j);
    p[j] = i;
    r[i] += (r[i] == r[j]);
  }
};
int main() {
  doin();
  int n, m;
  vi v;
  dsu c;
  cin >> n >> m;
  getv1(v, n);
  c.init(n);
  for (int a, b, i = 0; i < m; i++) {
    cin >> a >> b;
    c.unite(v[a], v[b]);
  }
  int ans = 0;
  for (int i = 1; i <= n; i++) {
    if (c.same(i, v[i])) {
      ans++;
    }
  }
  cout << ans;
  return 0;
}


Fixed submission:
#pragma GCC optimize("O3")
// #pragma GCC target ("sse4")
// #pragma GCC target ("avx2,tune=native")
#include <bits/stdc++.h>
// #include<ext/pb_ds/tree_policy.hpp>
// #include<ext/pb_ds/assoc_container.hpp>
#define pb push_back
#define __V vector
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define oit ostream_iterator
#define mod 1000000007ll
using namespace std;
void doin() {
  cin.tie();
  cout.tie();
  ios::sync_with_stdio(0);
#ifndef ONLINE_JUDGE
//	freopen("input.txt", "r", stdin);
//	freopen("output.txt", "w", stdout);
#endif
}
template <typename X, typename Y>
istream &operator>>(istream &in, pair<X, Y> &a) {
  in >> a.first >> a.second;
  return in;
}
template <typename T> void getv(T &i) { cin >> i; }
template <typename T, typename... Ns> void getv(vector<T> &v, int n, Ns... ns) {
  v.resize(n);
  for (auto &i : v)
    getv(i, ns...);
}
template <typename T> void getv1(T &i) { cin >> i; }
template <typename T, typename... Ns>
void getv1(vector<T> &v, int n, Ns... ns) {
  v.resize(n + 1);
  for (int i = 1; i <= n; i++)
    getv1(v[i], ns...);
}
#ifdef _WIN32
#define getchar_unlocked() _getchar_nolock()
#define _CRT_DISABLE_PERFCRIT_LOCKS
#endif
inline void getch(char &x) {
  while (x = getchar_unlocked(), x < 33) {
    ;
  }
}
inline void getstr(string &str) {
  str.clear();
  char cur;
  while (cur = getchar_unlocked(), cur < 33) {
    ;
  }
  while (cur > 32) {
    str += cur;
    cur = getchar_unlocked();
  }
}
template <typename T> inline bool sc(T &num) {
  bool neg = 0;
  int c;
  num = 0;
  while (c = getchar_unlocked(), c < 33) {
    if (c == EOF)
      return false;
  }
  if (c == '-') {
    neg = 1;
    c = getchar_unlocked();
  }
  for (; c > 47; c = getchar_unlocked())
    num = num * 10 + c - 48;
  if (neg)
    num *= -1;
  return true;
}
template <typename a, typename b> void minq(a &x, b y) {
  if (x > y)
    x = y;
}
template <typename a, typename b> void maxq(a &x, b y) {
  if (x < y)
    x = y;
}
typedef unsigned long long ull;
typedef long long ll;
typedef float ld;
typedef ll _I;
typedef pair<_I, _I> pi;
typedef pair<ld, ld> pd;
typedef map<_I, _I> mii;
typedef __V<_I> vi;
typedef __V<char> vc;
typedef __V<ld> vd;
typedef __V<vd> vvd;
typedef __V<pi> vpi;
typedef __V<__V<_I>> vvi;
typedef __V<__V<char>> vvc;
typedef __V<__V<pi>> vvpi;
using AntonTsypko = void;
using arsijo = AntonTsypko;
using god = arsijo;
uniform_real_distribution<double> double_dist(0, 1);
// WHEN DOING MODULAR SUBTRACTION ALWAYS **ALWAYS** ADD MOD
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
struct dsu {
  vi p, r;
  void init(int n) {
    r.assign(n + 1, 0);
    p.resize(n + 1);
    for (int i = 0; i <= n; i++)
      p[i] = i;
  }
  int par(int i) { return p[i] == i ? i : p[i] = par(p[i]); }
  bool same(int i, int j) { return par(i) == par(j); }
  void unite(int i, int j) {
    i = par(i), j = par(j);
    if (i == j)
      return;
    if (r[i] < r[j])
      swap(i, j);
    p[j] = i;
    r[i] += (r[i] == r[j]);
  }
};
int main() {
  doin();
  int n, m;
  vi v;
  dsu c;
  cin >> n >> m;
  getv1(v, n);
  c.init(n);
  for (int a, b, i = 0; i < m; i++) {
    cin >> a >> b;
    c.unite(v[a], v[b]);
  }
  int ans = 0;
  for (int i = 1; i <= n; i++) {
    if (c.same(i, v[i])) {
      ans++;
    }
  }
  cout << ans;
  return 0;
}