/*  Apply Kahn's algorithm to obtain a topological sorting of the original graph. */
vector<int> topologicalSort(vector<vector<tuple<int,int,int>>> graph)
{
    queue<int> Z;
    vector<int> topological_sort;
    int visited_nodes = 0;

    vector<int> indegree = getIndegree(graph);

    for (int i = 0; i < graph.size(); i++)
    {
        tuple<int,int,int> tuple = graph[i][0];

        if(indegree[i] == 0)
            Z.push(get<0>(tuple) - 1);
    }

    while (!Z.empty())
    {
        int vertex = Z.front();
        //cout << vertex;
        Z.pop();
        topological_sort.push_back(vertex);

        for (int i = 1; i < graph[vertex].size(); i++)
        {
            tuple<int,int,int> tuple = graph[vertex][i];
            int idx = get<0>(tuple) - 1;

            indegree[idx] -= 1;

            if(indegree[idx] == 0)
            {
                Z.push(idx);
            }
        }

        visited_nodes++;
    }

    if(visited_nodes !=  graph.size())
    {
        cout << "There is a cycle!" << endl;
        //return vector<int>(0);
    }

    return topological_sort;
}