DFS norecursion

#include <iostream>
#include <vector>

using namespace std;
int n, m;
vector <vector<int>> g;
vector <bool> used;
vector <int> q;

int main() {
    cin >> n >> m;
    g.resize(n);
    used.resize(n, false);
    for (int i = 0; i < m; i++){
        int x, y;
        cin >> x >> y;
        x--; y--;
        g[x].push_back(y);
        g[y].push_back(x);
    }
    q.push_back(0);
    used[0] = true;
    while(!q.empty()){
        int v = q.back();
        bool fl = true;
        int j = 0;
        while (j < g[v].size()){
            if (!used[g[v][j]]){
                fl = false;
                break;
            }
            else j++;
        }

        if (!fl){
            q.push_back(g[v][j]);
            cout << v+1 <<' '<<g[v][j]+1 <<'\n';
            used[g[v][j]] = true;
        }
        else q.pop_back();
    }
    return 0;
}