Grupuri și grupulețe

#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <queue>

struct Node {
    std::vector<int> neighbors;
};

class ListGraph {
private:
    std::vector<Node> nodes;
    std::vector<bool> visited;
    int size;

public:
    ListGraph(int sz) {
        /* Nodes are numbered 1 to N, not 0 to N. */
        size = sz;
        nodes.resize(sz + 1);
        for (int i = 0; i < sz + 1; i++)
            visited.push_back(false);
    }

    ~ListGraph() {}

    void addEdge(int src, int dst) {
        nodes[src].neighbors.push_back(dst);
    }

    /* you might want to write your own compare function.. check the problem statement */
    static bool compare_groups(std::vector<int> v1, std::vector<int> v2) {
        /* TODO: compare 2 groups properly */
        return v1[0] < v2[0];
    }

    /* get all groups in the classroom */
    void dfs(int nod, std::vector<bool> &viz, std::vector<int> &bis) {
        bis.push_back(nod);
        viz[nod] = 1;
        for (int i = 0; i < nodes[nod].neighbors.size(); i++) {
            if (viz[nodes[nod].neighbors[i]] == 0) {
                dfs(nodes[nod].neighbors[i], viz, bis);
            }
        }
    }
    std::vector<std::vector<int> > get_groups() {
        std::vector<std::vector<int> > bisericute;
        /* TODO: do your magic */
        int nr = 0;
        for (int i = 1; i <= size; i++) {
            if (visited[i] == false) {
                bisericute.push_back({});
                dfs(i, visited, bisericute[nr]);
                std::sort(bisericute[nr].begin(), bisericute[nr].end());
                nr++;
            }
        }
        /* TODO: return 'bisericute' sorted as described in the problem statement */
        std::sort(bisericute.begin(), bisericute.end(), compare_groups);
        return bisericute;
    }

};

int main() {
    /* DO NOT CHANGE ANYTHING HERE! */
    int N, M, from, to;
    std::vector<std::vector<int> > bisericute;

    std::cin >> N >> M;
    ListGraph g(N);
    for (int i = 0; i < M; i++) {
        std::cin >> from >> to;
        g.addEdge(from, to);
        g.addEdge(to, from);
    }

    bisericute = g.get_groups();
    std::cout << bisericute.size() << std::endl;
    for (int i = 0; i < bisericute.size(); i++) {
        for (int j = 0; j < bisericute[i].size(); j++) {
            std::cout << bisericute[i][j] << " ";
        }
        std::cout << std::endl;
    }

    return 0;
}