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

#include <bits/stdc++.h>
using namespace std;
using lint = long long int;

struct Graph {
  struct Edge {
    int from;
    int to;
    lint cost;
  };
  struct Node {
    Node() : par(-1), done(false), dist(INF) {}
    int index;
    int par;
    bool done;
    lint dist;
    vector<Edge> edge;
  };

  int n;
  vector<Node> nodes;
  vector<Edge> edges;
  const static lint INF = 1e18;

  Graph(int n) : n(n) {
    nodes.resize(n);
    for (int i = 0; i < n; i++)
      nodes[i].index = i;
  }

  void add_edge(int from, int to, lint cost) {
    nodes[from].edge.push_back(Edge{from, to, cost});
    edges.push_back(Edge{from, to, cost});
  }

  void dijkstra(int s) {
    using Cand = tuple<lint, int, int>; // (dist, index, from)
    priority_queue<Cand, vector<Cand>, greater<Cand>> que;
    que.emplace(0, s, -1);

    while (!que.empty()) {
      lint dist;
      int index, from;
      tie(dist, index, from) = que.top();
      que.pop();
      if (dist >= nodes[index].dist)
        continue;

      nodes[index].dist = dist;
      nodes[index].par = from;
      for (auto &e : nodes[index].edge) {
        que.emplace(dist + e.cost, e.to, e.from);
      }
    }
  }

  bool bellman_ford(int s) {
    bool negative_cycle;
    nodes[s].dist = 0;
    for (int step = 1; step <= n; step++) {
      bool update = false;
      for (auto &e : edges) {
        if (nodes[e.from].dist == INF)
          continue;

        lint next_cost = nodes[e.from].dist + e.cost;
        if (nodes[e.to].dist > next_cost) {
          nodes[e.to].dist = next_cost;
          nodes[e.to].par = e.from;
          update = true;
        }
      }
      negative_cycle = (step == n && update);
    }
    return negative_cycle;
  }

  void bfs(int s) {
    queue<int> que;
    nodes[s].dist = 0;
    que.push(s);

    while (!que.empty()) {
      int index = que.front();
      que.pop();
      for (auto &e : nodes[index].edge) {
        if (nodes[e.to].dist != INF)
          continue;
        nodes[e.to].dist = nodes[index].dist + 1;
        nodes[e.to].par = index;
        que.push(e.to);
      }
    }
  }

  lint shortest_dist(int to) { return nodes[to].dist; }

  vector<int> shortest_path(int to) {
    vector<int> path;
    int index = to;
    while (index != -1) {
      path.push_back(index);
      index = nodes[index].par;
    }
    reverse(path.begin(), path.end());
    return path;
  }
};

int main() {
  lint N, M;
  cin >> N >> M;

  Graph G(N);
  for (int i = 0; i < M; i++) {
    int a, b;
    cin >> a >> b;
    a--, b--;

    G.add_edge(a, b, 1);
    G.add_edge(b, a, 1);
  }

  G.bfs(0);
  cout << "Yes" << endl;
  for (int i = 1; i < N; i++) {
    cout << G.nodes[i].par + 1 << endl;
  }
}

Error: TLE

Fixed submission:

#include <bits/stdc++.h>
using namespace std;
using lint = long long int;

struct Graph {
  struct Edge {
    int from;
    int to;
    lint cost;
  };
  struct Node {
    Node() : par(-1), done(false), dist(INF) {}
    int index;
    int par;
    bool done;
    lint dist;
    vector<Edge> edge;
  };

  int n;
  vector<Node> nodes;
  vector<Edge> edges;
  const static lint INF = 1e18;

  Graph(int n) : n(n) {
    nodes.resize(n);
    for (int i = 0; i < n; i++)
      nodes[i].index = i;
  }

  void add_edge(int from, int to, lint cost) {
    nodes[from].edge.push_back(Edge{from, to, cost});
    edges.push_back(Edge{from, to, cost});
  }

  void dijkstra(int s) {
    using Cand = tuple<lint, int, int>; // (dist, index, from)
    priority_queue<Cand, vector<Cand>, greater<Cand>> que;
    que.emplace(0, s, -1);

    while (!que.empty()) {
      lint dist;
      int index, from;
      tie(dist, index, from) = que.top();
      que.pop();
      if (dist >= nodes[index].dist)
        continue;

      nodes[index].dist = dist;
      nodes[index].par = from;
      for (auto &e : nodes[index].edge) {
        que.emplace(dist + e.cost, e.to, e.from);
      }
    }
  }

  bool bellman_ford(int s) {
    bool negative_cycle;
    nodes[s].dist = 0;
    for (int step = 1; step <= n; step++) {
      bool update = false;
      for (auto &e : edges) {
        if (nodes[e.from].dist == INF)
          continue;

        lint next_cost = nodes[e.from].dist + e.cost;
        if (nodes[e.to].dist > next_cost) {
          nodes[e.to].dist = next_cost;
          nodes[e.to].par = e.from;
          update = true;
        }
      }
      negative_cycle = (step == n && update);
    }
    return negative_cycle;
  }

  void bfs(int s) {
    queue<int> que;
    nodes[s].dist = 0;
    que.push(s);

    while (!que.empty()) {
      int index = que.front();
      que.pop();
      for (auto &e : nodes[index].edge) {
        if (nodes[e.to].dist != INF)
          continue;
        nodes[e.to].dist = nodes[index].dist + 1;
        nodes[e.to].par = index;
        que.push(e.to);
      }
    }
  }

  lint shortest_dist(int to) { return nodes[to].dist; }

  vector<int> shortest_path(int to) {
    vector<int> path;
    int index = to;
    while (index != -1) {
      path.push_back(index);
      index = nodes[index].par;
    }
    reverse(path.begin(), path.end());
    return path;
  }
};

int main() {
  lint N, M;
  cin >> N >> M;

  Graph G(N);
  for (int i = 0; i < M; i++) {
    int a, b;
    cin >> a >> b;
    a--, b--;

    G.add_edge(a, b, 1);
    G.add_edge(b, a, 1);
  }

  G.bfs(0);
  cout << "Yes" << endl;
  for (int i = 1; i < N; i++) {
    auto v = G.shortest_path(i);
    cout << v[v.size() - 2] + 1 << endl;
  }
}