G6: Eulerian cycle

#include <iostream>
#include <fstream>
#include <vector>

using namespace std;

void printCycle(vector<vector<int>> &g, int v, ofstream &fout) {
    while (!g[v].empty()) {
        int tmp = g[v].back();
        g[v].pop_back();
        int i;
        for (i = 0; i < g[tmp].size() && g[tmp][i] != v; i++);
        g[tmp].erase(g[tmp].begin() + i);

        printCycle(g, tmp, fout);
    }

    fout << v + 1 << " ";
}

int main() {

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

    int n, m, tmpV1, tmpV2;
    fin >> n >> m;
    vector<vector<int>> g(n);

    for (int i = 0; i < m; i++) {
        fin >> tmpV1 >> tmpV2;
        g[tmpV1 - 1].push_back(tmpV2 - 1);
        g[tmpV2 - 1].push_back(tmpV1 - 1);
    }

    for (int i = 0; i < n; i++) {
        if (g[i].size() % 2 != 0) {
            fout << -1;
            return 0;
        }
    }

    printCycle(g, 0, fout);
}