Beams_of_arcs_list(not oriented)

#define _CRT_SECURE_NO_WARNINGS

#include <iostream>
#include <vector>

using namespace std;

class Graph
{
public:
    const int block = -1;
    int vcount, acount;
    vector<int> IJ, H, L;

    Graph(int _vcount, vector<int>& _i, vector<int>& _j) :vcount(_vcount), acount(_i.size() * 2) {
        IJ.resize(acount);
        for (int k(0); k < acount / 2; ++k) {
            IJ[k] = _i[k];
            int ind = acount - k - 1;
            IJ[ind] = _j[k];
        }
        makeList();
    }

    void makeList() {
        H.assign(vcount, block);
        L.resize(acount);
        for (int i(0); i < acount; ++i) {
            int ind(IJ[i]);
            L[i] = H[ind];
            H[ind] = i;
        }
    }

    void addArc(int i, int j) {
        IJ.insert(IJ.begin() + acount / 2, i);
        IJ.insert(IJ.begin() + acount / 2 + 1, j);
        acount += 2;
        makeList();
    }

    void print() {
        cout << "graph G {" << endl;
        for (int k(0); k < acount / 2; ++k) {
            cout << IJ[k] + 1 << " -- " << IJ[acount - k - 1] + 1<< ";" << endl;
        }
        cout << "}";
    }

    void showBeamsOfArcs() {
        cout << "Beams of arcs: " << endl;
        vector <int> ArcsList;
        for (int i(0); i < vcount; ++i) {
            ArcsList.clear();
            for (int k(H[i]); k != -1; ) {
                ArcsList.push_back(k >= acount / 2 ? acount - k - 1 : k);
                k = L[k];
            }

            int count(ArcsList.size());

            switch (count)
            {
            case 0:
                cout << "From vertex " << i + 1 << " come no edges";
                break;
            case 1:
                cout << "From vertex " << i + 1 << " comes 1 edge: ";
                break;
            default:
                cout << "From vertex " << i + 1 << " come " << count << " edges: ";
            }

            if (count > 0) {
                for (int k(0); k < count - 1; ++k) {
                    cout << ArcsList[k] + 1 << ", ";
                }
                cout << ArcsList[count - 1] + 1;
            }

            cout << endl;
        }
    }
};

int main()
{
    freopen("test.txt", "r", stdin);
    freopen("test.gv", "w", stdout);

    int n, m;
    cin >> n >> m;

    vector<int> _i, _j;
    int ii, jj;
    for (int i(0); i < m; ++i) {
        cin >> ii >> jj;
        _i.push_back(ii - 1);
        _j.push_back(jj - 1);
    }

    Graph G(n, _i, _j);
    G.print();

    return 0;
}