Problem p03805 (C++)

Buggy submission (status: Runtime Error):
#ifndef LOCAL
#pragma GCC optimize("O3")
#endif
#include "bits/stdc++.h"
#include <ext/pb_ds/assoc_container.hpp>
using namespace __gnu_pbds;
using namespace std;
#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__) << "] "
#define fastio                                                                 \
  ios_base::sync_with_stdio(false);                                            \
  cin.tie(0);
typedef long long ll;
typedef tree<int, null_type, less<int>, rb_tree_tag,
             tree_order_statistics_node_update>
    indexed_set;

int main() {
  fastio int n, m;
  cin >> n >> m;
  vector<vector<int>> v(n, vector<int>(m));
  for (int i = 0; i < n; i++) {
    int x, y;
    cin >> x >> y;
    x--, y--;
    v[x][y] = 1;
    v[y][x] = 1;
  }
  vector<int> a;
  for (int i = 1; i < n; i++) {
    a.push_back(i);
  }
  int ans = 0;
  do {
    a.insert(a.begin(), 0);
    bool ok = true;
    for (int i = 1; i < (int)a.size(); i++) {
      if (!v[a[i]][a[i - 1]])
        ok = false;
    }
    a.erase(a.begin());
    if (ok)
      ans++;
  } while (next_permutation(a.begin(), a.end()));
  cout << ans << endl;
  return 0;
}

Fixed submission:
#ifndef LOCAL
#pragma GCC optimize("O3")
#endif
#include "bits/stdc++.h"
#include <ext/pb_ds/assoc_container.hpp>
using namespace __gnu_pbds;
using namespace std;
#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__) << "] "
#define fastio                                                                 \
  ios_base::sync_with_stdio(false);                                            \
  cin.tie(0);
typedef long long ll;
typedef tree<int, null_type, less<int>, rb_tree_tag,
             tree_order_statistics_node_update>
    indexed_set;

int main() {
  fastio int n, m;
  cin >> n >> m;
  vector<vector<int>> v(n, vector<int>(m));
  for (int i = 0; i < m; i++) {
    int x, y;
    cin >> x >> y;
    x--, y--;
    v[x][y] = 1;
    v[y][x] = 1;
  }
  vector<int> a;
  for (int i = 1; i < n; i++) {
    a.push_back(i);
  }
  int ans = 0;
  do {
    a.insert(a.begin(), 0);
    bool ok = true;
    for (int i = 1; i < (int)a.size(); i++) {
      if (!v[a[i]][a[i - 1]])
        ok = false;
    }
    a.erase(a.begin());
    if (ok)
      ans++;
  } while (next_permutation(a.begin(), a.end()));
  cout << ans << endl;
  return 0;
}