Problem p03746 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>

const double pi = 3.141592653589793238462643383279;
using namespace std;
// typedef
//------------------------------------------
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef pair<long long, long long> PLL;
typedef pair<int, PII> TIII;
typedef long long LL;
typedef unsigned long long ULL;
typedef vector<LL> VLL;
typedef vector<VLL> VVLL;

// container util
//------------------------------------------
#define ALL(a) (a).begin(), (a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define SQ(a) ((a) * (a))
#define EACH(i, c)                                                             \
  for (typeof((c).begin()) i = (c).begin(); i != (c).end(); ++i)
#define EXIST(s, e) ((s).find(e) != (s).end())
#define SORT(c) sort((c).begin(), (c).end())

// repetition
//------------------------------------------
#define FOR(i, s, n) for (int i = s; i < (int)n; ++i)
#define REP(i, n) FOR(i, 0, n)
#define MOD 1000000007

#define rep(i, a, b) for (int i = a; i < (b); ++i)
#define trav(a, x) for (auto &a : x)
#define all(x) x.begin(), x.end()
#define sz(x) (int)(x).size()

typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;

#define chmin(x, y) x = min(x, y)
#define chmax(x, y) x = max(x, y)

// ここから編集

vector<vector<int>> g(100010);
bool used[100010];
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(10);

  int n, m;
  cin >> n >> m;
  REP(i, m) {
    int a, b;
    cin >> a >> b;
    a--;
    b--;
    g[a].push_back(b);
    g[b].push_back(a);
  }

  vector<int> ans;
  int st = 0;
  while (!g[st].size())
    st++;

  used[st] = true;
  ans.push_back(st);
  for (int now = st, bf = -1; now != bf;) {
    bf = now;
    for (auto u : g[now]) {
      if (used[u])
        continue;
      used[u] = true;
      ans.push_back(u);
      now = u;
      break;
    }
  }
  reverse(all(ans));
  for (int now = st, bf = -1; now != bf;) {
    bf = now;
    for (auto u : g[now]) {
      if (used[u])
        continue;
      used[u] = true;
      ans.push_back(u);
      now = u;
      break;
    }
  }

  cout << ans.size() << endl;
  for (int i = 0; i < n; i++) {
    if (i)
      cout << " ";
    cout << ans[i] + 1;
  }
  cout << endl;
  return 0;
}


Fixed submission:
#include <bits/stdc++.h>

const double pi = 3.141592653589793238462643383279;
using namespace std;
// typedef
//------------------------------------------
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef pair<long long, long long> PLL;
typedef pair<int, PII> TIII;
typedef long long LL;
typedef unsigned long long ULL;
typedef vector<LL> VLL;
typedef vector<VLL> VVLL;

// container util
//------------------------------------------
#define ALL(a) (a).begin(), (a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define SQ(a) ((a) * (a))
#define EACH(i, c)                                                             \
  for (typeof((c).begin()) i = (c).begin(); i != (c).end(); ++i)
#define EXIST(s, e) ((s).find(e) != (s).end())
#define SORT(c) sort((c).begin(), (c).end())

// repetition
//------------------------------------------
#define FOR(i, s, n) for (int i = s; i < (int)n; ++i)
#define REP(i, n) FOR(i, 0, n)
#define MOD 1000000007

#define rep(i, a, b) for (int i = a; i < (b); ++i)
#define trav(a, x) for (auto &a : x)
#define all(x) x.begin(), x.end()
#define sz(x) (int)(x).size()

typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;

#define chmin(x, y) x = min(x, y)
#define chmax(x, y) x = max(x, y)

// ここから編集

vector<vector<int>> g(100010);
bool used[100010];
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(10);

  int n, m;
  cin >> n >> m;
  REP(i, m) {
    int a, b;
    cin >> a >> b;
    a--;
    b--;
    g[a].push_back(b);
    g[b].push_back(a);
  }

  vector<int> ans;
  int st = 0;
  while (!g[st].size())
    st++;

  used[st] = true;
  ans.push_back(st);
  for (int now = st, bf = -1; now != bf;) {
    bf = now;
    for (auto u : g[now]) {
      if (used[u])
        continue;
      used[u] = true;
      ans.push_back(u);
      now = u;
      break;
    }
  }
  reverse(all(ans));
  for (int now = st, bf = -1; now != bf;) {
    bf = now;
    for (auto u : g[now]) {
      if (used[u])
        continue;
      used[u] = true;
      ans.push_back(u);
      now = u;
      break;
    }
  }

  cout << ans.size() << endl;
  for (int i = 0; i < ans.size(); i++) {
    if (i)
      cout << " ";
    cout << ans[i] + 1;
  }
  cout << endl;
  return 0;
}