Problem p02536 (C++)

Buggy submission (status: Runtime Error):
#line 1 "main.cpp"
#define _CRT_SECURE_NO_WARNINGS
#pragma GCC target("avx")
#pragma GCC optimize("O3")
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cfloat>
#include <climits>
#include <cmath>
#include <complex>
#include <ctime>
#include <deque>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <list>
#include <map>
#include <memory>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string.h>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
#define rep(i, n) for (int i = 0; i < (lint)(n); i++)
#define REP(i, n) for (int i = 1; i <= (lint)(n); i++)
#define all(V) V.begin(), V.end()
#define stackReplaceBegin(size)                                                \
  void *p = malloc(size);                                                      \
  esp_new = (lint)p + size - 1;                                                \
  void *stack_extend_memory_ = malloc(size);                                   \
  void *stack_extend_origin_memory_;                                           \
  char *stack_extend_dummy_memory_ = (char *)alloca(                           \
      (1 + (int)(((long long)stack_extend_memory_) & 127)) * 16);              \
  *stack_extend_dummy_memory_ = 0;                                             \
  asm volatile("mov %%rsp, %%rbx\nmov %%rax, %%rsp"                            \
               : "=b"(stack_extend_origin_memory_)                             \
               : "a"((char *)stack_extend_memory_ + (size)-1024));
typedef long long lint;
typedef unsigned long long ulint;
typedef std::pair<int, int> P;
constexpr int INF = INT_MAX / 2;
constexpr lint LINF = LLONG_MAX / 2;
constexpr double eps = DBL_EPSILON;
constexpr double PI = 3.141592653589793238462643383279;
lint esp_new;
template <class T>
class prique : public std::priority_queue<T, std::vector<T>, std::greater<T>> {
};
template <class T, class U> inline bool chmax(T &lhs, const U &rhs) {
  if (lhs < rhs) {
    lhs = rhs;
    return 1;
  }
  return 0;
}
template <class T, class U> inline bool chmin(T &lhs, const U &rhs) {
  if (lhs > rhs) {
    lhs = rhs;
    return 1;
  }
  return 0;
}
inline lint gcd(lint a, lint b) {
  while (b) {
    lint c = a;
    a = b;
    b = c % b;
  }
  return a;
}
inline lint lcm(lint a, lint b) { return a / gcd(a, b) * b; }
bool isprime(lint n) {
  if (n == 1)
    return false;
  for (int i = 2; i * i <= n; i++) {
    if (n % i == 0)
      return false;
  }
  return true;
}
template <typename T> T mypow(T a, unsigned int b) {
  T res(1), now(a);
  while (b) {
    if (b & 1)
      res *= now;
    b >>= 1;
    now *= now;
  }
  return res;
}
lint modpow(lint a, unsigned int b, unsigned int mod) {
  lint res(1), now(a % mod);
  while (b) {
    if (b & 1) {
      res *= now;
      res %= mod;
    }
    b >>= 1;
    now *= now;
    now %= mod;
  }
  return res;
}
template <typename T> void printArray(std::vector<T> &vec) {
  rep(i, (int)vec.size() - 1) std::cout << vec[i] << " ";
  if (!vec.empty())
    std::cout << vec.back() << std::endl;
}
template <typename T> void printArray(T l, T r) {
  T rprev = r;
  rprev--;
  for (T i = l; i != rprev; i++) {
    std::cout << *i << " ";
  }
  std::cout << *rprev << std::endl;
}
int n, m;
std::vector<int> vec[30010];
bool used[30010];
void dfs(int node) {
  used[node] = true;
  for (int i : vec[node]) {
    if (!used[i])
      dfs(i);
  }
}
int main() {
  std::cin >> n >> m;
  rep(i, m) {
    int a, b;
    std::cin >> a >> b;
    vec[a].emplace_back(b);
    vec[b].emplace_back(a);
  }
  int ans = 0;
  REP(i, n) {
    if (!used[i]) {
      dfs(i);
      ans++;
    }
  }
  std::cout << ans - 1 << std::endl;
  return 0;
}

Fixed submission:
#line 1 "main.cpp"
#define _CRT_SECURE_NO_WARNINGS
#pragma GCC target("avx")
#pragma GCC optimize("O3")
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cfloat>
#include <climits>
#include <cmath>
#include <complex>
#include <ctime>
#include <deque>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <list>
#include <map>
#include <memory>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string.h>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
#define rep(i, n) for (int i = 0; i < (lint)(n); i++)
#define REP(i, n) for (int i = 1; i <= (lint)(n); i++)
#define all(V) V.begin(), V.end()
#define stackReplaceBegin(size)                                                \
  void *p = malloc(size);                                                      \
  esp_new = (lint)p + size - 1;                                                \
  void *stack_extend_memory_ = malloc(size);                                   \
  void *stack_extend_origin_memory_;                                           \
  char *stack_extend_dummy_memory_ = (char *)alloca(                           \
      (1 + (int)(((long long)stack_extend_memory_) & 127)) * 16);              \
  *stack_extend_dummy_memory_ = 0;                                             \
  asm volatile("mov %%rsp, %%rbx\nmov %%rax, %%rsp"                            \
               : "=b"(stack_extend_origin_memory_)                             \
               : "a"((char *)stack_extend_memory_ + (size)-1024));
typedef long long lint;
typedef unsigned long long ulint;
typedef std::pair<int, int> P;
constexpr int INF = INT_MAX / 2;
constexpr lint LINF = LLONG_MAX / 2;
constexpr double eps = DBL_EPSILON;
constexpr double PI = 3.141592653589793238462643383279;
lint esp_new;
template <class T>
class prique : public std::priority_queue<T, std::vector<T>, std::greater<T>> {
};
template <class T, class U> inline bool chmax(T &lhs, const U &rhs) {
  if (lhs < rhs) {
    lhs = rhs;
    return 1;
  }
  return 0;
}
template <class T, class U> inline bool chmin(T &lhs, const U &rhs) {
  if (lhs > rhs) {
    lhs = rhs;
    return 1;
  }
  return 0;
}
inline lint gcd(lint a, lint b) {
  while (b) {
    lint c = a;
    a = b;
    b = c % b;
  }
  return a;
}
inline lint lcm(lint a, lint b) { return a / gcd(a, b) * b; }
bool isprime(lint n) {
  if (n == 1)
    return false;
  for (int i = 2; i * i <= n; i++) {
    if (n % i == 0)
      return false;
  }
  return true;
}
template <typename T> T mypow(T a, unsigned int b) {
  T res(1), now(a);
  while (b) {
    if (b & 1)
      res *= now;
    b >>= 1;
    now *= now;
  }
  return res;
}
lint modpow(lint a, unsigned int b, unsigned int mod) {
  lint res(1), now(a % mod);
  while (b) {
    if (b & 1) {
      res *= now;
      res %= mod;
    }
    b >>= 1;
    now *= now;
    now %= mod;
  }
  return res;
}
template <typename T> void printArray(std::vector<T> &vec) {
  rep(i, (int)vec.size() - 1) std::cout << vec[i] << " ";
  if (!vec.empty())
    std::cout << vec.back() << std::endl;
}
template <typename T> void printArray(T l, T r) {
  T rprev = r;
  rprev--;
  for (T i = l; i != rprev; i++) {
    std::cout << *i << " ";
  }
  std::cout << *rprev << std::endl;
}
int n, m;
std::vector<int> vec[100010];
bool used[100010];
void dfs(int node) {
  used[node] = true;
  for (int i : vec[node]) {
    if (!used[i])
      dfs(i);
  }
}
int main() {
  std::cin >> n >> m;
  rep(i, m) {
    int a, b;
    std::cin >> a >> b;
    vec[a].emplace_back(b);
    vec[b].emplace_back(a);
  }
  int ans = 0;
  REP(i, n) {
    if (!used[i]) {
      dfs(i);
      ans++;
    }
  }
  std::cout << ans - 1 << std::endl;
  return 0;
}