Problem p02678 (C++) — buggy submission, verdict: Time Limit Exceeded

// #define LOCAL
#ifdef LOCAL
#define _GLIBCXX_DEBUG
#endif
#include <bits/stdc++.h>
using namespace std;
#define rep(i, x) for (ll i = 0; i < (ll)(x); i++)
#define rrep(i, x) for (ll i = (ll)(x)-1; 0 <= i; i--)
#define reps(i, x) for (ll i = 1; i < (ll)(x) + 1; i++)
#define rreps(i, x) for (ll i = (ll)(x); 1 <= i; i--)
#define all(x) (x).begin(), (x).end()
typedef long long ll;
typedef long double ld;
typedef pair<ll, ll> Pll;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<vvl> vvvl;
constexpr ll INF = numeric_limits<ll>::max() / 4;
constexpr ll n_max = 2e5 + 10;
#define int ll

template <typename A, typename B> string to_string(pair<A, B> p);
string to_string(const string &s) { return '"' + s + '"'; }
string to_string(const char c) { return to_string((string)&c); }
string to_string(bool b) { return (b ? "true" : "false"); }
template <size_t N> string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++)
    res += static_cast<char>('0' + v[i]);
  return res;
}
template <typename A> string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first)
      res += ", ";
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}
template <typename A, typename B> string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

void debug_out() { cerr << endl; }
template <typename Head, typename... Tail> void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}

#ifdef LOCAL
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif

template <typename T> struct edge {
  int f, t;
  T c;
  int id;
  edge(){};
  edge(int f, int t, T c, int id = 0) : f(f), t(t), c(c), id(id){};
  bool operator<(const edge &rhs) const { return (*this).c < rhs.c; }
  bool operator>(const edge &rhs) const { return (*this).c > rhs.c; }
};

template <typename T> struct graph {
  std::vector<std::vector<edge<T>>> data;
  graph(int v) : data(v){};
  void add_edge(edge<T> &e) { data[e.f].push_back(e); }
  void add_edge(int f, int t, T c) { data[f].emplace_back(f, t, c); }
  size_t size() { return data.size(); }
  vector<edge<T>> operator[](int n) { return data[n]; }
  std::vector<edge<T>> make_edges() {
    std::vector<edge<T>> r;
    for (auto &i : data)
      std::copy(i.begin(), i.end(), std::back_inserter(r));
    return r;
  }
};

template <typename T> std::vector<T> dijkstra(graph<T> &g, int s) {
  using state = std::pair<T, int>;
  // priority_queue
  std::priority_queue<state, std::vector<state>, std::greater<state>> q;
  q.emplace((T)0, s);
  // data init
  std::vector<T> data(g.size(), std::numeric_limits<T>::max());
  data[s] = (T)0;
  // solve
  while (!q.empty()) {
    state cur = q.top();
    q.pop();
    T c = cur.first;
    int pos = cur.second;
    if (data[pos] < c)
      continue;
    for (auto &e : g.data[pos]) {
      if (c + e.c < data[e.t]) {
        data[e.t] = c + e.c;
        q.emplace(c + e.c, e.t);
      }
    }
  }
  return data;
};
void YES(bool ok) { cout << (ok ? "Yes" : "No") << endl; }

signed main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);

  ll n, m;
  cin >> n >> m;
  graph<ll> g(n);
  vector<Pll> vec;
  rep(i, m) {
    ll a, b;
    cin >> a >> b;
    a--;
    b--;
    g.add_edge(a, b, 1);
    g.add_edge(b, a, 1);
    vec.emplace_back(a, b);
  }

  auto dij = dijkstra(g, 0);

  vector<ll> ans(n, -1);
  bool ok = true;
  rep(i, n) { ok &= dij[i] != numeric_limits<ll>::max(); }
  // rep(i,n){
  //     for(auto &e : g[i]){
  //         if(ans[e.f] != -1 && ans[e.t] == -1)continue;
  //         if(dij[e.f] + 1 == dij[e.t]){
  //             ans[e.t] = e.f + 1;
  //         }
  //         if(dij[e.t] + 1 == dij[e.f]){
  //             ans[e.f] = e.t + 1;
  //         }
  //     }
  // }

  YES(ok);
  if (ok) {

    rep(i, m) {
      auto [f, t] = vec[i];
      debug(f, t);
      if (ans[f] != -1 && ans[t] != -1)
        continue;
      if (dij[f] + 1 == dij[t]) {
        ans[t] = f + 1;
        continue;
      }
      if (dij[t] + 1 == dij[f]) {
        ans[f] = t + 1;
      }
    }
    reps(i, n - 1) { cout << ans[i] << "\\n"; }
  }
}

Error: TLE

Fixed submission:

#define LOCAL
#ifdef LOCAL
#define _GLIBCXX_DEBUG
#endif
#include <bits/stdc++.h>
using namespace std;
#define rep(i, x) for (ll i = 0; i < (ll)(x); i++)
#define rrep(i, x) for (ll i = (ll)(x)-1; 0 <= i; i--)
#define reps(i, x) for (ll i = 1; i < (ll)(x) + 1; i++)
#define rreps(i, x) for (ll i = (ll)(x); 1 <= i; i--)
#define all(x) (x).begin(), (x).end()
typedef long long ll;
typedef long double ld;
typedef pair<ll, ll> Pll;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<vvl> vvvl;
constexpr ll INF = numeric_limits<ll>::max() / 4;
constexpr ll n_max = 2e5 + 10;
#define int ll

template <typename A, typename B> string to_string(pair<A, B> p);
string to_string(const string &s) { return '"' + s + '"'; }
string to_string(const char c) { return to_string((string)&c); }
string to_string(bool b) { return (b ? "true" : "false"); }
template <size_t N> string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++)
    res += static_cast<char>('0' + v[i]);
  return res;
}
template <typename A> string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first)
      res += ", ";
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}
template <typename A, typename B> string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

void debug_out() { cerr << endl; }
template <typename Head, typename... Tail> void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}

#ifdef LOCAL
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif

template <typename T> struct edge {
  int f, t;
  T c;
  int id;
  edge(){};
  edge(int f, int t, T c, int id = 0) : f(f), t(t), c(c), id(id){};
  bool operator<(const edge &rhs) const { return (*this).c < rhs.c; }
  bool operator>(const edge &rhs) const { return (*this).c > rhs.c; }
};

template <typename T> struct graph {
  std::vector<std::vector<edge<T>>> data;
  graph(int v) : data(v){};
  void add_edge(edge<T> &e) { data[e.f].push_back(e); }
  void add_edge(int f, int t, T c) { data[f].emplace_back(f, t, c); }
  size_t size() { return data.size(); }
  vector<edge<T>> operator[](int n) { return data[n]; }
  std::vector<edge<T>> make_edges() {
    std::vector<edge<T>> r;
    for (auto &i : data)
      std::copy(i.begin(), i.end(), std::back_inserter(r));
    return r;
  }
};

template <typename T> std::vector<T> dijkstra(graph<T> &g, int s) {
  using state = std::pair<T, int>;
  // priority_queue
  std::priority_queue<state, std::vector<state>, std::greater<state>> q;
  q.emplace((T)0, s);
  // data init
  std::vector<T> data(g.size(), std::numeric_limits<T>::max());
  data[s] = (T)0;
  // solve
  while (!q.empty()) {
    state cur = q.top();
    q.pop();
    T c = cur.first;
    int pos = cur.second;
    if (data[pos] < c)
      continue;
    for (auto &e : g.data[pos]) {
      if (c + e.c < data[e.t]) {
        data[e.t] = c + e.c;
        q.emplace(c + e.c, e.t);
      }
    }
  }
  return data;
};
void YES(bool ok) { cout << (ok ? "Yes" : "No") << endl; }

signed main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);

  ll n, m;
  cin >> n >> m;
  graph<ll> g(n);
  rep(i, m) {
    ll a, b;
    cin >> a >> b;
    a--;
    b--;
    g.add_edge(a, b, 1);
    g.add_edge(b, a, 1);
  }

  auto dij = dijkstra(g, 0);

  vector<ll> ans(n, -1);
  bool ok = true;
  rep(i, n) { ok &= dij[i] != numeric_limits<ll>::max(); }
  rep(i, n) {
    for (auto &e : g[i]) {
      if (ans[e.f] != -1 && ans[e.t] == -1)
        continue;
      if (dij[e.f] + 1 == dij[e.t]) {
        ans[e.t] = e.f + 1;
      }
      if (dij[e.t] + 1 == dij[e.f]) {
        ans[e.f] = e.t + 1;
      }
    }
  }
  YES(ok);
  if (ok) {
    reps(i, n - 1) { cout << ans[i] << "\\n"; }
  }
}