Problem p03993 (C++)

Buggy submission (status: Runtime Error):
#pragma gcc optimize("Ofast")
#include <bits/stdc++.h>
using namespace std;

#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define REP(i, n) for (int i = 0; i < (n); ++i)
#define PER(i, n) for (int i = (n - 1); i >= 0; --i)
#define ALL(V) (V).begin(), (V).end()
#define SORT(V) sort(ALL(V))   // 小さい方からソート
#define REV(V) reverse(ALL(V)) // リバース
#define RSORT(V)                                                               \
  SORT(V);                                                                     \
  REV(V)                                 // 大きい方からソート
#define NEXP(V) next_permutation(ALL(V)) // 順列
#define pb(n) push_back(n)
#define popb(n) pop_back(n)
#define endl '\n'
#define Endl '\n'
#define DUMP(x) cout << #x << " = " << (x) << endl
#define YES(n) cout << ((n) ? "YES" : "NO") << endl
#define Yes(n) cout << ((n) ? "Yes" : "No") << endl
#define yes(n) cout << ((n) ? "yes" : "no") << endl
#define Yay(n) cout << ((n) ? "Yay!" : ":(") << endl
#define VSUM(V) accumulate(ALL(V), 0)
#define MID(a, b, c) (a) < (b) && (b) < (c)
#define MIDe(a, b, c) (a) <= (b) && (b) <= (c)
#define IN(n) cin >> n
#define IN2(a, b) cin >> a >> b
#define IN3(a, b, c) cin >> a >> b >> c
#define IN4(a, b, c, d) cin >> a >> b >> c >> d
#define VIN(V)                                                                 \
  for (int i = 0; i < (V).size(); i++) {                                       \
    cin >> (V).at(i);                                                          \
  }
#define OUT(n) cout << n << endl
#define VOUT(V)                                                                \
  REP(i, (V).size()) { cout << (V)[i] << endl; }
#define VOUT2(V)                                                               \
  REP(i, (V).size()) {                                                         \
    if ((V).size() - 1 != i) {                                                 \
      cout << (V)[i] << " ";                                                   \
    } else {                                                                   \
      cout << (V)[i] << endl;                                                  \
    }                                                                          \
  }

#define int long long
#define P pair<ll, ll>
#define Vi vector<ll>
#define VVi vector<vector<ll>>
#define Vd vector<double>
#define Vb vector<bool>
#define Vs vector<string>
#define Vc vector<char>
#define M map<ll, ll>
#define S set<ll>
#define PQ priority_queue<ll>
#define PQG priority_queue < ll, V, greater<ll>

using ll = long long;
using Graph = vector<vector<int>>;

const int MOD = 1000000007;
const int INF = 1061109567;
const double EPS = 1e-10;
const double PI = acos(-1.0);
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};

class UnionFind {
public:
  vector<int> par;
  vector<int> siz;

  UnionFind(int sz_) : par(sz_), siz(sz_, 1LL) {
    for (int i = 0; i < sz_; ++i)
      par[i] = i;
  }
  void init(ll sz_) {
    par.resize(sz_);
    siz.assign(sz_, 1LL);
    for (int i = 0; i < sz_; ++i)
      par[i] = i;
  }

  int root(int x) {
    while (par[x] != x) {
      x = par[x] = par[par[x]];
    }
    return x;
  }

  bool merge(int x, int y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (siz[x] < siz[y])
      swap(x, y);
    siz[x] += siz[y];
    par[y] = x;
    return true;
  }

  bool issame(int x, int y) { return root(x) == root(y); }

  int size(int x) { return siz[root(x)]; }
};

int gcd(int a, int b) { return b != 0 ? gcd(b, a % b) : a; }

int lcm(int a, int b) { return a * b / gcd(a, b); }

// 文字を全て大文字にします
string toStrUp(string str) {
  char diff = 'A' - 'a';
  REP(i, str.size()) str[i] += diff;
  return str;
}

// 文字をstring型で一文字取得します
string get1ch(string str, int key) { return str.substr(key, 1); }

// 素因数分解 (O(sqrt(n)))
map<int, int> prime_factor(int n) {
  map<int, int> ret;
  for (int i = 2; i * i <= n; i++) {
    while (n % i == 0) {
      ret[i]++;
      n /= i;
    }
  }
  if (n != 1)
    ret[n] = 1;
  return ret;
}

// 素数判定 (O(sqrt(n)))
bool is_prime(int x) {
  for (int i = 2; i * i <= x; i++) {
    if (x % i == 0)
      return false;
  }
  return true;
}

// 進数変換 (O(log n))
template <typename T> vector<T> convert_base(T x, T b) {
  vector<T> ret;
  T t = 1, k = abs(b);
  while (x) {
    ret.emplace_back((x * t) % k);
    if (ret.back() < 0)
      ret.back() += k;
    x -= ret.back() * t;
    x /= k;
    t *= b / k;
  }
  if (ret.empty())
    ret.emplace_back(0);
  reverse(begin(ret), end(ret));
  return ret;
}

template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}

template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}

int modPow(int a, int n) {
  if (n == 1)
    return a % MOD;
  if (n % 2 == 1)
    return (a * modPow(a, n - 1)) % MOD;
  int t = modPow(a, n / 2);
  return (t * t) % MOD;
}

signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(15);

  // デフォルト変数定義
  int n = 0, m = 0, a = 0, b = 0, c = 0, d = 0, x = 0, y = 0, z = 0;
  string s = "", t = "";
  //

  // ここから
  IN(n);
  Vi A(n);
  VIN(A)
  REP(i, n) {
    if (A[A[i - 1]] - 1 == i)
      x++;
  }
  OUT(x / 2);
}

Fixed submission:
#pragma gcc optimize("Ofast")
#include <bits/stdc++.h>
using namespace std;

#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define REP(i, n) for (int i = 0; i < (n); ++i)
#define PER(i, n) for (int i = (n - 1); i >= 0; --i)
#define ALL(V) (V).begin(), (V).end()
#define SORT(V) sort(ALL(V))   // 小さい方からソート
#define REV(V) reverse(ALL(V)) // リバース
#define RSORT(V)                                                               \
  SORT(V);                                                                     \
  REV(V)                                 // 大きい方からソート
#define NEXP(V) next_permutation(ALL(V)) // 順列
#define pb(n) push_back(n)
#define popb(n) pop_back(n)
#define endl '\n'
#define Endl '\n'
#define DUMP(x) cout << #x << " = " << (x) << endl
#define YES(n) cout << ((n) ? "YES" : "NO") << endl
#define Yes(n) cout << ((n) ? "Yes" : "No") << endl
#define yes(n) cout << ((n) ? "yes" : "no") << endl
#define Yay(n) cout << ((n) ? "Yay!" : ":(") << endl
#define VSUM(V) accumulate(ALL(V), 0)
#define MID(a, b, c) (a) < (b) && (b) < (c)
#define MIDe(a, b, c) (a) <= (b) && (b) <= (c)
#define IN(n) cin >> n
#define IN2(a, b) cin >> a >> b
#define IN3(a, b, c) cin >> a >> b >> c
#define IN4(a, b, c, d) cin >> a >> b >> c >> d
#define VIN(V)                                                                 \
  for (int i = 0; i < (V).size(); i++) {                                       \
    cin >> (V).at(i);                                                          \
  }
#define OUT(n) cout << n << endl
#define VOUT(V)                                                                \
  REP(i, (V).size()) { cout << (V)[i] << endl; }
#define VOUT2(V)                                                               \
  REP(i, (V).size()) {                                                         \
    if ((V).size() - 1 != i) {                                                 \
      cout << (V)[i] << " ";                                                   \
    } else {                                                                   \
      cout << (V)[i] << endl;                                                  \
    }                                                                          \
  }

#define int long long
#define P pair<ll, ll>
#define Vi vector<ll>
#define VVi vector<vector<ll>>
#define Vd vector<double>
#define Vb vector<bool>
#define Vs vector<string>
#define Vc vector<char>
#define M map<ll, ll>
#define S set<ll>
#define PQ priority_queue<ll>
#define PQG priority_queue < ll, V, greater<ll>

using ll = long long;
using Graph = vector<vector<int>>;

const int MOD = 1000000007;
const int INF = 1061109567;
const double EPS = 1e-10;
const double PI = acos(-1.0);
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};

class UnionFind {
public:
  vector<int> par;
  vector<int> siz;

  UnionFind(int sz_) : par(sz_), siz(sz_, 1LL) {
    for (int i = 0; i < sz_; ++i)
      par[i] = i;
  }
  void init(ll sz_) {
    par.resize(sz_);
    siz.assign(sz_, 1LL);
    for (int i = 0; i < sz_; ++i)
      par[i] = i;
  }

  int root(int x) {
    while (par[x] != x) {
      x = par[x] = par[par[x]];
    }
    return x;
  }

  bool merge(int x, int y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (siz[x] < siz[y])
      swap(x, y);
    siz[x] += siz[y];
    par[y] = x;
    return true;
  }

  bool issame(int x, int y) { return root(x) == root(y); }

  int size(int x) { return siz[root(x)]; }
};

int gcd(int a, int b) { return b != 0 ? gcd(b, a % b) : a; }

int lcm(int a, int b) { return a * b / gcd(a, b); }

// 文字を全て大文字にします
string toStrUp(string str) {
  char diff = 'A' - 'a';
  REP(i, str.size()) str[i] += diff;
  return str;
}

// 文字をstring型で一文字取得します
string get1ch(string str, int key) { return str.substr(key, 1); }

// 素因数分解 (O(sqrt(n)))
map<int, int> prime_factor(int n) {
  map<int, int> ret;
  for (int i = 2; i * i <= n; i++) {
    while (n % i == 0) {
      ret[i]++;
      n /= i;
    }
  }
  if (n != 1)
    ret[n] = 1;
  return ret;
}

// 素数判定 (O(sqrt(n)))
bool is_prime(int x) {
  for (int i = 2; i * i <= x; i++) {
    if (x % i == 0)
      return false;
  }
  return true;
}

// 進数変換 (O(log n))
template <typename T> vector<T> convert_base(T x, T b) {
  vector<T> ret;
  T t = 1, k = abs(b);
  while (x) {
    ret.emplace_back((x * t) % k);
    if (ret.back() < 0)
      ret.back() += k;
    x -= ret.back() * t;
    x /= k;
    t *= b / k;
  }
  if (ret.empty())
    ret.emplace_back(0);
  reverse(begin(ret), end(ret));
  return ret;
}

template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}

template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}

int modPow(int a, int n) {
  if (n == 1)
    return a % MOD;
  if (n % 2 == 1)
    return (a * modPow(a, n - 1)) % MOD;
  int t = modPow(a, n / 2);
  return (t * t) % MOD;
}

signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(15);

  // デフォルト変数定義
  int n = 0, m = 0, a = 0, b = 0, c = 0, d = 0, x = 0, y = 0, z = 0;
  string s = "", t = "";
  //

  // ここから
  IN(n);
  Vi A(n);
  VIN(A)
  REP(i, n) {
    if (A[A[i] - 1] - 1 == i)
      x++;
  }
  OUT(x / 2);
}