Problem p03356 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bitset>
#include <ciso646>
#include <cmath>
#include <complex>
#include <cstdio>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <utility>
#include <vector>
using namespace std;
typedef long long ll;
typedef unsigned int ui;
const ll mod = 1000000007;
typedef long double ld;
const ll INF = 1e+14;
typedef pair<int, int> P;
#define stop                                                                   \
  char nyaa;                                                                   \
  cin >> nyaa;
#define rep(i, n) for (int i = 0; i < n; i++)
#define per(i, n) for (int i = n - 1; i >= 0; i--)
#define Rep(i, sta, n) for (int i = sta; i < n; i++)
#define rep1(i, n) for (int i = 1; i <= n; i++)
#define per1(i, n) for (int i = n; i >= 1; i--)
#define Rep1(i, sta, n) for (int i = sta; i <= n; i++)
typedef complex<ld> Point;
const ld eps = 1e-8;
const ld pi = acos(-1.0);
typedef pair<ld, ld> LDP;
typedef pair<ll, ll> LP;
#define fr first
#define sc second

int par[100100];
int rannk[100100];

void init(int n) {
  rep(i, n) {
    par[i] = i;
    rannk[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 (rannk[x] < rannk[y]) {
    par[x] = y;
  } else {
    par[y] = x;
    if (rannk[x] == rannk[y])
      rannk[x]++;
  }
}

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

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

  int N, M;
  cin >> N >> M;
  init(N);
  vector<int> p;
  rep(i, N) {
    int q;
    cin >> q;
    q--;
    p[i] = q;
  }
  rep(i, M) {
    int x, y;
    cin >> x >> y;
    x--;
    y--;
    unite(p[x], p[y]);
  }
  int ans = 0;
  rep(i, N) {
    if (same(i, p[i]))
      ans++;
  }
  cout << ans << "\n";
  return 0;
}

Fixed submission:
#include <algorithm>
#include <bitset>
#include <ciso646>
#include <cmath>
#include <complex>
#include <cstdio>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <utility>
#include <vector>
using namespace std;
typedef long long ll;
typedef unsigned int ui;
const ll mod = 1000000007;
typedef long double ld;
const ll INF = 1e+14;
typedef pair<int, int> P;
#define stop                                                                   \
  char nyaa;                                                                   \
  cin >> nyaa;
#define rep(i, n) for (int i = 0; i < n; i++)
#define per(i, n) for (int i = n - 1; i >= 0; i--)
#define Rep(i, sta, n) for (int i = sta; i < n; i++)
#define rep1(i, n) for (int i = 1; i <= n; i++)
#define per1(i, n) for (int i = n; i >= 1; i--)
#define Rep1(i, sta, n) for (int i = sta; i <= n; i++)
typedef complex<ld> Point;
const ld eps = 1e-8;
const ld pi = acos(-1.0);
typedef pair<ld, ld> LDP;
typedef pair<ll, ll> LP;
#define fr first
#define sc second

int par[100100];
int rannk[100100];

void init(int n) {
  rep(i, n) {
    par[i] = i;
    rannk[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 (rannk[x] < rannk[y]) {
    par[x] = y;
  } else {
    par[y] = x;
    if (rannk[x] == rannk[y])
      rannk[x]++;
  }
}

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

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

  int N, M;
  cin >> N >> M;
  init(N);
  vector<int> p;
  rep(i, N) {
    int q;
    cin >> q;
    q--;
    p.push_back(q);
  }
  rep(i, M) {
    int x, y;
    cin >> x >> y;
    x--;
    y--;
    unite(p[x], p[y]);
  }
  int ans = 0;
  rep(i, N) {
    if (same(i, p[i]))
      ans++;
  }
  cout << ans << "\n";
  return 0;
}