lessons/参考实现/01-欧拉路径/AtCoder-agc032_c.cpp

#include <bits/stdc++.h>
using namespace std;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int n, m;
    cin >> n >> m;
    vector<vector<int>> graph(n);
    vector<int> degree(n, 0);
    for (int i = 0; i < m; ++i) {
        int u, v;
        cin >> u >> v;
        --u;
        --v;
        graph[u].push_back(v);
        graph[v].push_back(u);
        ++degree[u];
        ++degree[v];
    }
    for (int d : degree) {
        if (d & 1) {
            cout << "No\n";
            return 0;
        }
    }
    for (int d : degree) {
        if (d >= 6) {
            cout << "Yes\n";
            return 0;
        }
    }

    vector<int> four;
    for (int v = 0; v < n; ++v) {
        if (degree[v] == 4) four.push_back(v);
    }
    if (four.size() >= 3) {
        cout << "Yes\n";
        return 0;
    }
    if (four.size() <= 1) {
        cout << "No\n";
        return 0;
    }

    const int start = four[0], forbidden = four[1];
    vector<char> visited(n, false);
    stack<pair<int, int>> work;
    for (int to : graph[start]) {
        if (to != forbidden) work.push({to, start});
    }
    visited[start] = true;
    bool found_cycle_avoiding_other = false;
    while (!work.empty()) {
        auto [v, parent] = work.top();
        work.pop();
        if (v == start) {
            found_cycle_avoiding_other = true;
            break;
        }
        if (v == forbidden || visited[v]) continue;
        visited[v] = true;
        for (int to : graph[v]) {
            if (to != parent) work.push({to, v});
        }
    }
    cout << (found_cycle_avoiding_other ? "Yes\n" : "No\n");
    return 0;
}