Problem p02678 (C++) — buggy submission, verdict: Runtime Error

#define _GLIBCXX_DEBUG
#include <algorithm> //next_permutation
#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); i++)
#define all(v) v.begin(), v.end()
#define dec(n) cout << fixed << setprecision(n);
#define large "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
#define small "abcdefghijklmnopqrstuvwxyz"
using namespace std;
using ll = long long;
using P = pair<ll, ll>;
using vl = vector<ll>;
using vvl = vector<vl>;

ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}

const ll MOD = 1000000007;
const ll MAX = 2000001;

ll mod(ll a) { return a % MOD; }

ll lcm(ll a, ll b) { return (a * b) / gcd(a, b); }

ll fac[MAX], finv[MAX], inv[MAX];

void nCrprep() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (ll i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}

ll nCr(ll n, ll r) {
  if (n < r)
    return 0;
  if (n < 0 || r < 0)
    return 0;
  return fac[n] * (finv[r] * finv[n - r] % MOD) % MOD;
}

ll nCrcheep(ll n, ll r) {
  if (r == 0)
    return 1;
  else if (r == 1)
    return n;
  else
    return nCrcheep(n - 1, r - 1) * n / r;
}

vector<pair<ll, ll>> prime_factorize(ll n) {
  vector<pair<ll, ll>> res;

  for (ll i = 2; i * i <= n; i++) {
    if (n % i != 0)
      continue;

    ll ex = 0;
    while (n % i == 0) {
      ex++;
      n /= i;
    }

    res.push_back({i, ex});
  }

  if (n != 1)
    res.push_back({n, 1});

  return res;
}

int main() {
  ll n, m;
  cin >> n >> m;
  vector<bool> iscame(n + 1, false);
  vector<vector<ll>> connect(n + 1);
  ll cnt = 1;
  vl roomnum(n + 1);

  rep(i, m) {
    ll a, b;
    cin >> a >> b;
    connect[a].push_back(b);
    connect[b].push_back(a);
  }

  queue<ll> q;
  q.push(1);

  while (cnt <= n) {
    ll x = q.front();
    q.pop();

    for (ll room : connect[x]) {
      if (!iscame[room]) {
        cnt++;
        iscame[room] = true;
        roomnum[room] = x;
        q.push(room);
      }
    }
  }

  cout << "Yes" << endl;

  for (ll i = 2; i <= n; i++) {
    cout << roomnum[i] << endl;
  }
}

Error: 0

Fixed submission:

#define _GLIBCXX_DEBUG
#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); i++)
#define all(v) v.begin(), v.end()
#define dec(n) cout << fixed << setprecision(n);
#define large "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
#define small "abcdefghijklmnopqrstuvwxyz"
using namespace std;
using ll = long long;
using P = pair<ll, ll>;
using vl = vector<ll>;
using vvl = vector<vl>;

ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}

ll mod(ll a) { return (a % 1000000007); }

ll lcm(ll a, ll b) { return (a * b) / gcd(a, b); }

int main() {
  ll n, m;
  cin >> n >> m;
  vector<vector<ll>> table(n, vector<ll>(n));
  vector<ll> post(n, -1);

  rep(i, m) {
    ll x, y;
    cin >> x >> y;
    x--;
    y--;
    table[x][y] = 1;
    table[y][x] = 1;
  }

  queue<ll> explo;

  explo.push(0);

  post[0] = 0;

  while (!explo.empty()) {
    ll nowroom = explo.front();
    explo.pop();

    rep(i, n) {
      if (table[nowroom][i] == 1 && post[i] == -1) {
        post[i] = nowroom + 1;
        explo.push(i);
      }
    }
  }

  cout << "Yes" << endl;

  rep(i, n - 1) { cout << post[i + 1] << endl; }
}