Code : Always Late

#include <bits/stdc++.h>
using namespace std;

const int inf = (int)1e7;

class Graph {
  public:
    int n;
    vector<vector<pair<int, int>>> adj;
    Graph(int n) {
        this->n = n;
        adj.resize(n);
    }

  public:
    void dijkstra(int src, int dest);
};

void Graph ::dijkstra(int src, int dest) {
    vector<pair<int, int>> wave_arrival_time(n, {inf, inf});
    vector<int> visited_cnt(n);
    wave_arrival_time[src] = {0, inf};

    // At every event, a wave reaches a vertex for the first or second time.
    // Hence, we only need to run this algorithm for 2*n times.
    for (int i = 0; i < 2 * n; i++) {
        int vertex;
        int current_time = inf;
        for (int v = 0; v < n; v++) {
            if (visited_cnt[v] == 0 &&
                wave_arrival_time[v].first < current_time) {
                current_time = wave_arrival_time[v].first;
                vertex = v;
            }
            if (visited_cnt[v] == 1 &&
                wave_arrival_time[v].second < current_time) {
                current_time = wave_arrival_time[v].second;
                vertex = v;
            }
        }

        visited_cnt[vertex]++;

        // Emit waves for all the neighbors.
        for (auto [child, weight] : adj[vertex]) {
            auto arrival_time_of_this_wave = current_time + weight;
            // If this wave is faster, the wave in queue might act as the
            // second wave to arrive at that vertex.
            if (wave_arrival_time[child].first > arrival_time_of_this_wave) {
                wave_arrival_time[child].second =
                    wave_arrival_time[child].first;
                wave_arrival_time[child].first = arrival_time_of_this_wave;
            } else if (wave_arrival_time[child].first ==
                       arrival_time_of_this_wave) {
                // Do nothing. We've just found a different shortest path.
            } else {
                if (wave_arrival_time[child].second >
                    arrival_time_of_this_wave) {
                    wave_arrival_time[child].second = arrival_time_of_this_wave;
                }
            }
        }
    }

    if (wave_arrival_time[dest].second >= inf) {
        cout << "?" << endl;
    } else {
        cout << wave_arrival_time[dest].second << endl;
    }
}

void solve(int n) {
    int m;
    cin >> m;

    Graph g(n);
    for (int i = 0; i < m; i++) {
        int x, y, w;
        cin >> x >> y >> w;
        g.adj[x].push_back({y, w});
        g.adj[y].push_back({x, w});
    }

    int q;
    cin >> q;
    for (int i = 0; i < q; i++) {
        int src, dest;
        cin >> src >> dest;
        g.dijkstra(src, dest);
    }
}

int main() {
    int n;
    int tc = 1;
    while (cin >> n) {
        cout << "Set #" << tc << endl;
        solve(n);
        tc++;
    }
    return 0;
}