Problem p03356 (C++)

Buggy submission (status: Runtime Error):
/* main code starts from line 130. */

/* ---------- STL Libraries ---------- */

// IO library
#include <cstdio>
#include <iomanip>
#include <ios>
#include <iostream>

// algorithm library
#include <algorithm>
#include <cmath>
#include <numeric>

// container library
#include <bitset>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <vector>

/* ---------- Namespace ---------- */

using namespace std;

/* ---------- Type Abbreviation ---------- */

template <typename T> using V = vector<T>;
template <typename T, typename U> using P = pair<T, U>;
template <typename T> using PQ = priority_queue<T>;

using ll = long long;
using str = string;

#define fst first
#define snd second
#define pb push_back
#define mp make_pair
#define mt make_tuple

/* ---------- conversion ---------- */

#define INT(c) static_cast<int>(c)
#define CHAR(n) static_cast<char>(n)
#define LL(n) static_cast<ll>(n)
#define DOUBLE(n) static_cast<double>(n)

/* ---------- container ---------- */

#define ALL(v) (v).begin(), (v).end()
#define SIZE(v) (LL((v).size()))

#define FIND(v, k) (v).find(k) != (v).end()
#define VFIND(v, k) find(ALL(v), k) != (v).end

#define SORT(v) sort(ALL(v))
#define GSORT(v) sort(ALL(v), greater<decltype((v).front())>())

/* ---------- repetition ---------- */

#define FOR(i, a, b) for (ll i = (a); i < (b); i++)
#define REP(i, n) FOR(i, 0, n)
#define NREP(i, n) FOR(i, 1, n + 1)

#define RFOR(i, a, b) for (ll i = (a); i >= (b); i--)
#define RREP(i, n) RFOR(i, n - 1, 0)
#define RNREP(i, n) RFOR(i, n, 1)

// Usual REP runs from 0 to n-1 (R: n-1 to 0)
// Natural REP runs from 1 to n (R: n to 1)

/* ---------- Short Functions ---------- */

template <typename T> T sq(T a) { return a * a; }

#define fcout cout << fixed << setprecision(10)

/* ----------- debug ---------- */

template <typename T, typename U> void testP2(T a, U b) {
  cout << "(" << a << ", " << b << ")" << endl;
  return;
}

template <typename T> void testV(T v) {
  cout << "[";
  for (auto i : v) {
    cout << i << ", ";
  }
  cout << "\b\b]" << endl;
  return;
}

template <typename T> void testV2(T v) {
  for (auto sv : v) {
    testV(sv);
  }
  cout << endl;
  return;
}

#define GET_VAR_NAME(variable) #variable
#define test(x) cout << GET_VAR_NAME(x) << " = " << x << endl;
#define testP(p)                                                               \
  cout << GET_VAR_NAME(p) << " = ";                                            \
  testP2(p.fst, p.snd);

/* ---------- Constants ---------- */

// const ll MOD = 1e9 + 7;
// const int INF = 1 << 25;
// const ll INF = 1LL << 50;
// const double PI = 3.14159265358979;
// const ll dx[4] = {0, -1, 1, 0};
// const ll dy[4] = {-1, 0, 0, 1};
// const ll dx[8] = {-1, 0, 1, -1, 1, -1, 0, 1};
// const ll dy[8] = {-1, -1, -1, 0, 0, 1, 1, 1};

/* v-v-v-v-v-v-v-v-v Main Part v-v-v-v-v-v-v-v-v */

/* ---------- Type Definition ----------- */

class UnionFind {
public:
  // コンストラクタ
  UnionFind(ll N) {
    V_NUM = N;
    REP(i, V_NUM) {
      // 初期化: par[i] = i, rank[i] = 0
      par.pb(i);
      rank.pb(0);
      num.pb(1);
    }
  }

  // xの親を返す+更新
  ll find(ll x) {
    if (par[x] == x) {
      return x;
    } else {
      return par[x] = find(par[x]);
    }
  }

  // xとyを含むグループを統合する
  void unite(ll x, ll y) {
    x = find(x);
    y = find(y);

    if (x == y)
      return;

    num[find(x)] += num[find(y)];
    num[find(y)] = num[find(x)];

    // rankの小さい方にくっつける
    if (rank[x] < rank[y]) {
      par[x] = y;
    } else {
      par[y] = x;
      if (rank[x] == rank[y])
        rank[x]++;
    }
  }

  // xとyが同じグループに属するか判定
  bool same(ll x, ll y) { return find(x) == find(y); }

  ll V_NUM;
  V<ll> par, rank, num;
};

/* ---------- Global Variance ----------- */

/* ------------- Functions -------------- */

/* ----------- Main Function ------------ */

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);

  ll N, M;
  cin >> N >> M;
  V<ll> p(N);
  UnionFind uf(N + 1);

  NREP(i, N) { cin >> p[i]; }

  REP(i, M) {
    ll x, y;
    cin >> x >> y;
    uf.unite(x, y);
  }

  ll ans = 0;
  NREP(i, N) {
    if (uf.find(i) == uf.find(p[i])) {
      ans++;
    }
  }

  cout << ans << endl;
  return 0;
}

Fixed submission:
/* main code starts from line 130. */

/* ---------- STL Libraries ---------- */

// IO library
#include <cstdio>
#include <iomanip>
#include <ios>
#include <iostream>

// algorithm library
#include <algorithm>
#include <cmath>
#include <numeric>

// container library
#include <bitset>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <vector>

/* ---------- Namespace ---------- */

using namespace std;

/* ---------- Type Abbreviation ---------- */

template <typename T> using V = vector<T>;
template <typename T, typename U> using P = pair<T, U>;
template <typename T> using PQ = priority_queue<T>;

using ll = long long;
using str = string;

#define fst first
#define snd second
#define pb push_back
#define mp make_pair
#define mt make_tuple

/* ---------- conversion ---------- */

#define INT(c) static_cast<int>(c)
#define CHAR(n) static_cast<char>(n)
#define LL(n) static_cast<ll>(n)
#define DOUBLE(n) static_cast<double>(n)

/* ---------- container ---------- */

#define ALL(v) (v).begin(), (v).end()
#define SIZE(v) (LL((v).size()))

#define FIND(v, k) (v).find(k) != (v).end()
#define VFIND(v, k) find(ALL(v), k) != (v).end

#define SORT(v) sort(ALL(v))
#define GSORT(v) sort(ALL(v), greater<decltype((v).front())>())

/* ---------- repetition ---------- */

#define FOR(i, a, b) for (ll i = (a); i < (b); i++)
#define REP(i, n) FOR(i, 0, n)
#define NREP(i, n) FOR(i, 1, n + 1)

#define RFOR(i, a, b) for (ll i = (a); i >= (b); i--)
#define RREP(i, n) RFOR(i, n - 1, 0)
#define RNREP(i, n) RFOR(i, n, 1)

// Usual REP runs from 0 to n-1 (R: n-1 to 0)
// Natural REP runs from 1 to n (R: n to 1)

/* ---------- Short Functions ---------- */

template <typename T> T sq(T a) { return a * a; }

#define fcout cout << fixed << setprecision(10)

/* ----------- debug ---------- */

template <typename T, typename U> void testP2(T a, U b) {
  cout << "(" << a << ", " << b << ")" << endl;
  return;
}

template <typename T> void testV(T v) {
  cout << "[";
  for (auto i : v) {
    cout << i << ", ";
  }
  cout << "\b\b]" << endl;
  return;
}

template <typename T> void testV2(T v) {
  for (auto sv : v) {
    testV(sv);
  }
  cout << endl;
  return;
}

#define GET_VAR_NAME(variable) #variable
#define test(x) cout << GET_VAR_NAME(x) << " = " << x << endl;
#define testP(p)                                                               \
  cout << GET_VAR_NAME(p) << " = ";                                            \
  testP2(p.fst, p.snd);

/* ---------- Constants ---------- */

// const ll MOD = 1e9 + 7;
// const int INF = 1 << 25;
// const ll INF = 1LL << 50;
// const double PI = 3.14159265358979;
// const ll dx[4] = {0, -1, 1, 0};
// const ll dy[4] = {-1, 0, 0, 1};
// const ll dx[8] = {-1, 0, 1, -1, 1, -1, 0, 1};
// const ll dy[8] = {-1, -1, -1, 0, 0, 1, 1, 1};

/* v-v-v-v-v-v-v-v-v Main Part v-v-v-v-v-v-v-v-v */

/* ---------- Type Definition ----------- */

class UnionFind {
public:
  // コンストラクタ
  UnionFind(ll N) {
    V_NUM = N;
    REP(i, V_NUM) {
      // 初期化: par[i] = i, rank[i] = 0
      par.pb(i);
      rank.pb(0);
      num.pb(1);
    }
  }

  // xの親を返す+更新
  ll find(ll x) {
    if (par[x] == x) {
      return x;
    } else {
      return par[x] = find(par[x]);
    }
  }

  // xとyを含むグループを統合する
  void unite(ll x, ll y) {
    x = find(x);
    y = find(y);

    if (x == y)
      return;

    num[find(x)] += num[find(y)];
    num[find(y)] = num[find(x)];

    // rankの小さい方にくっつける
    if (rank[x] < rank[y]) {
      par[x] = y;
    } else {
      par[y] = x;
      if (rank[x] == rank[y])
        rank[x]++;
    }
  }

  // xとyが同じグループに属するか判定
  bool same(ll x, ll y) { return find(x) == find(y); }

  ll V_NUM;
  V<ll> par, rank, num;
};

/* ---------- Global Variance ----------- */

/* ------------- Functions -------------- */

/* ----------- Main Function ------------ */

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);

  ll N, M;
  cin >> N >> M;
  V<ll> p(N + 1);
  UnionFind uf(N + 1);

  NREP(i, N) { cin >> p[i]; }

  REP(i, M) {
    ll x, y;
    cin >> x >> y;
    uf.unite(x, y);
  }

  ll ans = 0;
  NREP(i, N) {
    if (uf.find(i) == uf.find(p[i])) {
      ans++;
    }
  }

  cout << ans << endl;
  return 0;
}