This submission is migrated from previous version of oj.uz, which used different machine for grading. This submission may have different result if resubmitted.
#include <bits/stdc++.h>
using namespace std;
int n, m, u, v;
vector<queue<pair<int, int>>> adj;
vector<int> res;
vector<bool> sus;
void euler(int s) {
    while (!adj[s].empty()) {
        int e = adj[s].front().first;
        int i = adj[s].front().second;
        adj[s].pop();
        if (!sus[i]) { sus[i] = true; euler(e); }
        sus[i] = true;
    }
    res.push_back(s);
}
int main() {
    cin >> n >> m;
    adj.resize(n);
    sus.resize(m);
    vector<int> deg(n);
    int g = 0;
    for (int i = 0; i < m; i++) {
        cin >> u >> v;
        adj[u-1].push({v-1, g});
        adj[v-1].push({u-1, g++});
    }
    euler(0);
    reverse(res.begin(), res.end());
    vector<int> vst(n, -1);
    vector<int> nxt(n, -1);
    for (int i = 0; i < res.size(); i++) {
        int node = res[i];
        if (vst[node] == -1) {
            vst[node] = i;
            nxt[node] = res[i+1];
        }
        else {
            int nd = res[vst[node]];
            do {
                cout << nd+1 << ' ';
                vst[nd] = -1;
                nd = nxt[nd];
            } while (nd != node);
            cout << '\n';
            nxt[node] = res[i+1];
            vst[node] = i;
        }
    }
}
Compilation message (stderr)
postmen.cpp: In function 'int main()':
postmen.cpp:37:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   37 |     for (int i = 0; i < res.size(); i++) {
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