Problem p02536 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#include <iostream>
#include <stdio.h>
#include <stdlib.h>
using namespace std;
// #include <ext/pb_ds/assoc_container.hpp>
// #include <ext/pb_ds/tree_policy.hpp>
// using namespace __gnu_pbds;
/*---------------------DEBUGGING--------------------------------------------*/
void __print(int x) { cerr << x; }
void __print(long x) { cerr << x; }
void __print(long long x) { cerr << x; }
void __print(unsigned x) { cerr << x; }
void __print(unsigned long x) { cerr << x; }
void __print(unsigned long long x) { cerr << x; }
void __print(float x) { cerr << x; }
void __print(double x) { cerr << x; }
void __print(long double x) { cerr << x; }
void __print(char x) { cerr << '\'' << x << '\''; }
void __print(const char *x) { cerr << '\"' << x << '\"'; }
void __print(const string &x) { cerr << '\"' << x << '\"'; }
void __print(bool x) { cerr << (x ? "true" : "false"); }

template <typename T, typename V> void __print(const pair<T, V> &x) {
  cerr << '{';
  __print(x.first);
  cerr << ',';
  __print(x.second);
  cerr << '}';
}
template <typename T> void __print(const T &x) {
  int f = 0;
  cerr << '{';
  for (auto &i : x)
    cerr << (f++ ? "," : ""), __print(i);
  cerr << "}";
}
void _print() { cerr << "]\n"; }
template <typename T, typename... V> void _print(T t, V... v) {
  __print(t);
  if (sizeof...(v))
    cerr << ", ";
  _print(v...);
}
#ifndef ONLINE_JUDGE
#define debug(x...)                                                            \
  cerr << "[" << #x << "] = [";                                                \
  _print(x)
#else
#define debug(x...)
#endif
/*-------------------------------------------------------------------------------------*/
// #define mp make_pair
#define pb push_back
#define ll long long
#define pii pair<int, int>
#define pcc pair<char, char>
#define F first
#define S second
#define int long long
#define pi 3.141592653589793238462643383279502
#define M 1000000007 // 998244353
#define rep(i, a, n) for (int i = a; i < n; i++)
#define INF 100000000000000000
#define N 200005
#define vi vector<int>
#define all(v) v.begin(), v.end()
#define endl "\n"
// #define ordered_set tree<int, null_type,less<int>,
// rb_tree_tag,tree_order_statistics_node_update>
vector<int> adj[N];
int vis[N];
int dfs(int x) {
  vis[x] = 1;
  for (auto y : adj[x]) {
    if (!vis[y])
      dfs(y);
  }
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int n, m;
  cin >> n >> m;
  rep(i, 0, m) {
    int u, v;
    cin >> u >> v;
    adj[u].pb(v);
    adj[v].pb(u);
  }
  int cc = 0;
  for (int i = 1; i <= n; i++) {
    if (!vis[i]) {
      dfs(i);
      cc++;
    }
  }
  cout << (cc - 1) << endl;
}


Fixed submission:
#include <bits/stdc++.h>
#include <iostream>
#include <stdio.h>
#include <stdlib.h>
using namespace std;
// #include <ext/pb_ds/assoc_container.hpp>
// #include <ext/pb_ds/tree_policy.hpp>
// using namespace __gnu_pbds;
/*---------------------DEBUGGING--------------------------------------------*/
void __print(int x) { cerr << x; }
void __print(long x) { cerr << x; }
void __print(long long x) { cerr << x; }
void __print(unsigned x) { cerr << x; }
void __print(unsigned long x) { cerr << x; }
void __print(unsigned long long x) { cerr << x; }
void __print(float x) { cerr << x; }
void __print(double x) { cerr << x; }
void __print(long double x) { cerr << x; }
void __print(char x) { cerr << '\'' << x << '\''; }
void __print(const char *x) { cerr << '\"' << x << '\"'; }
void __print(const string &x) { cerr << '\"' << x << '\"'; }
void __print(bool x) { cerr << (x ? "true" : "false"); }

template <typename T, typename V> void __print(const pair<T, V> &x) {
  cerr << '{';
  __print(x.first);
  cerr << ',';
  __print(x.second);
  cerr << '}';
}
template <typename T> void __print(const T &x) {
  int f = 0;
  cerr << '{';
  for (auto &i : x)
    cerr << (f++ ? "," : ""), __print(i);
  cerr << "}";
}
void _print() { cerr << "]\n"; }
template <typename T, typename... V> void _print(T t, V... v) {
  __print(t);
  if (sizeof...(v))
    cerr << ", ";
  _print(v...);
}
#ifndef ONLINE_JUDGE
#define debug(x...)                                                            \
  cerr << "[" << #x << "] = [";                                                \
  _print(x)
#else
#define debug(x...)
#endif
/*-------------------------------------------------------------------------------------*/
// #define mp make_pair
#define pb push_back
#define ll long long
#define pii pair<int, int>
#define pcc pair<char, char>
#define F first
#define S second
#define int long long
#define pi 3.141592653589793238462643383279502
#define M 1000000007 // 998244353
#define rep(i, a, n) for (int i = a; i < n; i++)
#define INF 100000000000000000
#define N 200005
#define vi vector<int>
#define all(v) v.begin(), v.end()
#define endl "\n"
// #define ordered_set tree<int, null_type,less<int>,
// rb_tree_tag,tree_order_statistics_node_update>
vector<int> adj[N];
int vis[N];
void dfs(int x) {
  vis[x] = 1;
  for (auto y : adj[x]) {
    if (!vis[y])
      dfs(y);
  }
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int n, m;
  cin >> n >> m;
  rep(i, 0, m) {
    int u, v;
    cin >> u >> v;
    adj[u].pb(v);
    adj[v].pb(u);
  }
  int cc = 0;
  for (int i = 1; i <= n; i++) {
    if (!vis[i]) {
      dfs(i);
      cc++;
    }
  }
  cout << (cc - 1) << endl;
}