Problem p02536 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;

#define ll long long
#define X first
#define Y second
#define all(x) x.begin(), x.end()
#define ordered_set                                                            \
  tree<int, null_type, less<int>, rb_tree_tag,                                 \
       tree_order_statistics_node_update>

#define sim template <class c
#define ris return *this
#define dor > debug &operator<<
#define eni(x)                                                                 \
  sim > typename enable_if<sizeof dud<c>(0) x 1, debug &>::type operator<<(    \
            c i) {
sim > struct rge {
  c b, e;
};
sim > rge<c> range(c i, c j) { return rge<c>{i, j}; }
sim > auto dud(c *x) -> decltype(cerr << *x, 0);
sim > char dud(...);
struct debug {
#ifdef LOCAL
  ~debug() { cerr << endl; }
  eni(!=) cerr << boolalpha << i;
  ris;
} eni(==) ris << range(begin(i), end(i));
}
sim, class b dor(pair<b, c> d) {
  ris << "(" << d.first << ", " << d.second << ")";
}
sim dor(rge<c> d) {
  *this << "[";
  for (auto it = d.b; it != d.e; ++it)
    *this << ", " + 2 * (it == d.b) << *it;
  ris << "]";
}
#else
  sim dor(const c &) { ris; }
#endif
}
;
#define imie(...) " [" << #__VA_ARGS__ ": " << (__VA_ARGS__) << "] "

const int MOD = 998244353;
const int MX = (int)1e5 + 10;

void add_self(int &x, int y, int md = MOD) {
  x += y;
  if (x >= md)
    x -= md;
}
void sub_self(int &x, int y, int md = MOD) {
  x -= y;
  if (x < 0)
    x += md;
}
void mul_self(int &x, int y, int md = MOD) { x = x * 1LL * y % md; }
inline int mul(int x, int y, int md = MOD) { return x * 1LL * y % md; }
inline int add(int x, int y, int md = MOD) {
  x += y;
  if (x >= md)
    x -= md;
  return x;
}
inline int sub(int x, int y, int md = MOD) {
  x -= y;
  if (x < 0)
    x += md;
  return x;
}

const int dx[] = {0, 0, -1, 1};
const int dy[] = {1, -1, 0, 0};

vector<int> read(int n, int k) {
  vector<int> vec(n);
  for (int i = 0; i < k; ++i)
    cin >> vec[i];
  int a, b, c, d;
  cin >> a >> b >> c >> d;
  for (int i = k; i < n; ++i)
    vec[i] = (a * 1ll * vec[i - 2] + b * 1ll * vec[i - 1] + c) % d;
  return vec;
}

vector<int> g[MX], vis(MX);

void dfs(int x, int par = -1) {
  vis[x] = true;
  for (auto &y : g[x]) {
    if (y == par)
      continue;
    dfs(y, x);
  }
}

void solve() {
  int n, m;
  cin >> n >> m;
  for (int i = 0; i < m; ++i) {
    int x, y;
    cin >> x >> y;
    --x, --y;
    g[x].push_back(y);
    g[y].push_back(x);
  }
  int comp = 0;
  for (int i = 0; i < n; ++i) {
    if (!vis[i]) {
      dfs(i);
      comp++;
    }
  }
  cout << comp - 1 << "\n";
  return;
}

int main() {

  cin.tie(0);
  cout.tie(0);
  ios_base::sync_with_stdio(0);

  // freopen("input.in", "r", stdin);
  // freopen("output.out", "w", stdout);

  int t;
  t = 1;
  // cin >> t;
  // pre();
  for (int i = 1; i <= t; ++i) {
    // cout << "Case #" << i << ": ";
    solve();
    // cout << '\n';
    // cout << endl;
  }

  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;

#define ll long long
#define X first
#define Y second
#define all(x) x.begin(), x.end()
#define ordered_set                                                            \
  tree<int, null_type, less<int>, rb_tree_tag,                                 \
       tree_order_statistics_node_update>

#define sim template <class c
#define ris return *this
#define dor > debug &operator<<
#define eni(x)                                                                 \
  sim > typename enable_if<sizeof dud<c>(0) x 1, debug &>::type operator<<(    \
            c i) {
sim > struct rge {
  c b, e;
};
sim > rge<c> range(c i, c j) { return rge<c>{i, j}; }
sim > auto dud(c *x) -> decltype(cerr << *x, 0);
sim > char dud(...);
struct debug {
#ifdef LOCAL
  ~debug() { cerr << endl; }
  eni(!=) cerr << boolalpha << i;
  ris;
} eni(==) ris << range(begin(i), end(i));
}
sim, class b dor(pair<b, c> d) {
  ris << "(" << d.first << ", " << d.second << ")";
}
sim dor(rge<c> d) {
  *this << "[";
  for (auto it = d.b; it != d.e; ++it)
    *this << ", " + 2 * (it == d.b) << *it;
  ris << "]";
}
#else
  sim dor(const c &) { ris; }
#endif
}
;
#define imie(...) " [" << #__VA_ARGS__ ": " << (__VA_ARGS__) << "] "

const int MOD = 998244353;
const int MX = (int)1e5 + 10;

void add_self(int &x, int y, int md = MOD) {
  x += y;
  if (x >= md)
    x -= md;
}
void sub_self(int &x, int y, int md = MOD) {
  x -= y;
  if (x < 0)
    x += md;
}
void mul_self(int &x, int y, int md = MOD) { x = x * 1LL * y % md; }
inline int mul(int x, int y, int md = MOD) { return x * 1LL * y % md; }
inline int add(int x, int y, int md = MOD) {
  x += y;
  if (x >= md)
    x -= md;
  return x;
}
inline int sub(int x, int y, int md = MOD) {
  x -= y;
  if (x < 0)
    x += md;
  return x;
}

const int dx[] = {0, 0, -1, 1};
const int dy[] = {1, -1, 0, 0};

vector<int> read(int n, int k) {
  vector<int> vec(n);
  for (int i = 0; i < k; ++i)
    cin >> vec[i];
  int a, b, c, d;
  cin >> a >> b >> c >> d;
  for (int i = k; i < n; ++i)
    vec[i] = (a * 1ll * vec[i - 2] + b * 1ll * vec[i - 1] + c) % d;
  return vec;
}

vector<int> g[MX], vis(MX);

void dfs(int x, int par = -1) {
  vis[x] = true;
  for (auto &y : g[x]) {
    if (y == par)
      continue;
    if (!vis[y])
      dfs(y, x);
  }
}

void solve() {
  int n, m;
  cin >> n >> m;
  for (int i = 0; i < m; ++i) {
    int x, y;
    cin >> x >> y;
    --x, --y;
    g[x].push_back(y);
    g[y].push_back(x);
  }
  int comp = 0;
  for (int i = 0; i < n; ++i) {
    if (!vis[i]) {
      dfs(i);
      comp++;
    }
  }
  cout << comp - 1 << "\n";
  return;
}

int main() {

  cin.tie(0);
  cout.tie(0);
  ios_base::sync_with_stdio(0);

  // freopen("input.in", "r", stdin);
  // freopen("output.out", "w", stdout);

  int t;
  t = 1;
  // cin >> t;
  // pre();
  for (int i = 1; i <= t; ++i) {
    // cout << "Case #" << i << ": ";
    solve();
    // cout << '\n';
    // cout << endl;
  }

  return 0;
}