Problem p02536 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp> // Common file
#include <ext/pb_ds/detail/standard_policies.hpp>
#include <ext/pb_ds/tree_policy.hpp> // Including tree_order_statistics_node_update

/*
 *    coder :: ATUL_PANDEY_2608
 *        >>> INDIA <<<
 */
using namespace std;
using namespace __gnu_pbds;

// #define  part ..
#define pb(a) push_back(a)
#define all(a) a.begin(), a.end()
#define mod 1000000007
// #define maxx 200006
#define ll long long
#define quick ios_base::sync_with_stdio(NULL), cin.tie(0);
#define listll vector<long long>
#define listi vector<int>
#define pii pair<int, int>
#define pll pair<long long, long long>
#define minheap priority_queue<long long, vector<long long>, greater<long long>>
#define rep(i, a, b) for (int i = a; i < b; i++)

ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }

ll power(ll a, ll n, ll m = 1000000007) {
  ll ans = 1;
  while (n) {
    if (n & 1) {
      ans *= a;
      ans %= m;
    }
    a *= a;
    a %= m;
    n /= 2;
  }
  return ans;
}

vector<int> prm;
vector<bool> vsp;

void prime(int nn = 100000) {
  vsp = vector<bool>(nn + 1, 0);
  int size_prime = nn;
  for (ll i = 2; i * i < size_prime; i++) {

    if (vsp[i] == 1)
      continue;
    for (ll j = i * i; j < size_prime; j += i) {

      vsp[j] = 1;
    }
  }

  for (int i = 2; i < size_prime; i++)
    if (vsp[i] == 0)
      prm.pb(i);
}

template <class T>
using oset =
    tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;

// end of #define

// define globals ...
const int maxx = 2e5 + 26;

// write function from here ...
//  never forget  to recheck your predefined function in template ..
//  already defined ::  prime , power , gcd : ( ll )..

vector<int> adj[maxx];

vector<int> vis;
void dfs(int u, int par) {

  vis[u] = 1;
  if (adj[u].size() == 0)
    return;
  for (auto it : adj[u]) {

    if (it != par) {
      dfs((int)it, u);
    }
  }
};

int Main() {

  ll n, m;
  cin >> n >> m;
  for (int i = 0; i < m; i++) {
    int a, b;

    cin >> a >> b;
    a--, b--;
    adj[a].pb(b);
    adj[b].pb(a);
  }

  vis = vector<int>(maxx, 0);

  int cnt = 0;
  for (int i = 0; i < n; i++) {

    if (vis[i] == 0) {

      dfs(i, -1);
      cnt++;
    }
  }

  cout << cnt - 1 << endl;

  return 0;
}

int main() {
  quick;
  int t = 1;
  // cin>>t;
  while (t--)
    Main();

  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp> // Common file
#include <ext/pb_ds/detail/standard_policies.hpp>
#include <ext/pb_ds/tree_policy.hpp> // Including tree_order_statistics_node_update

/*
 *    coder :: ATUL_PANDEY_2608
 *        >>> INDIA <<<
 */
using namespace std;
using namespace __gnu_pbds;

// #define  part ..
#define pb(a) push_back(a)
#define all(a) a.begin(), a.end()
#define mod 1000000007
// #define maxx 200006
#define ll long long
#define quick ios_base::sync_with_stdio(NULL), cin.tie(0);
#define listll vector<long long>
#define listi vector<int>
#define pii pair<int, int>
#define pll pair<long long, long long>
#define minheap priority_queue<long long, vector<long long>, greater<long long>>
#define rep(i, a, b) for (int i = a; i < b; i++)

ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }

ll power(ll a, ll n, ll m = 1000000007) {
  ll ans = 1;
  while (n) {
    if (n & 1) {
      ans *= a;
      ans %= m;
    }
    a *= a;
    a %= m;
    n /= 2;
  }
  return ans;
}

vector<int> prm;
vector<bool> vsp;

void prime(int nn = 100000) {
  vsp = vector<bool>(nn + 1, 0);
  int size_prime = nn;
  for (ll i = 2; i * i < size_prime; i++) {

    if (vsp[i] == 1)
      continue;
    for (ll j = i * i; j < size_prime; j += i) {

      vsp[j] = 1;
    }
  }

  for (int i = 2; i < size_prime; i++)
    if (vsp[i] == 0)
      prm.pb(i);
}

template <class T>
using oset =
    tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;

// end of #define

// define globals ...
const int maxx = 2e5 + 26;

// write function from here ...
//  never forget  to recheck your predefined function in template ..
//  already defined ::  prime , power , gcd : ( ll )..

vector<int> adj[maxx];

vector<int> vis;
void dfs(int u, int par) {

  vis[u] = 1;
  if (adj[u].size() == 0)
    return;
  for (auto it : adj[u]) {

    if (it != par && vis[it] == 0) {
      dfs((int)it, u);
    }
  }
};

int Main() {

  ll n, m;
  cin >> n >> m;
  for (int i = 0; i < m; i++) {
    int a, b;

    cin >> a >> b;
    a--, b--;
    adj[a].pb(b);
    adj[b].pb(a);
  }

  vis = vector<int>(maxx, 0);

  int cnt = 0;
  for (int i = 0; i < n; i++) {

    if (vis[i] == 0) {

      dfs(i, -1);
      cnt++;
    }
  }

  cout << cnt - 1 << endl;

  return 0;
}

int main() {
  quick;
  int t = 1;
  // cin>>t;
  while (t--)
    Main();

  return 0;
}