modues c++

#include <fstream>
#include <vector>
#include <string>

using namespace std;


bool dfs(int v, vector<vector<int>> mat, vector<int> &used) {
    used[v] = 1;
    for (int i = 0; i < mat[v].size(); i++) {
        if (used[mat[v][i]] == 0) {
            dfs(mat[v][i], mat, used);
            used[v] = 2;
        }
    }
    used[v] = 2;
    return true;
}

int main() {
    ifstream fin("input.txt");
    ofstream fout("output.txt");

    int vertex, edges;
    fin >> vertex >> edges;
    vector<vector<int>> mat(vertex + 1, vector<int>());
    vector<int> used(vertex + 1);

    for (int i = 1; i < edges + 1; i++) {
        int a, b;
        fin >> a >> b;
        mat[a].push_back(b);
        mat[b].push_back(a);
    }

    int count = 0;
    for (int i = 1; i < used.size(); i++) {
        if (used[i] == 0) {
            dfs(i, mat, used);
            count++;
        }
    }

    fout << count;
    return 0;

}