9E-并查集

 #include <iostream>
#include <vector>
#define pii pair <int, int>
#define mp make_pair

using namespace std;

vector <vector <int> > G;
vector <bool> mark;
vector <pii> res;
bool cycle = 0;

void dfs(int u, int p = -1)
{
    mark[u] = 1;
    int i;

    for (i = 0; i < G[u].size(); i++)
    {
        if (!mark[G[u][i]])
            dfs(G[u][i], u);
        else if (G[u][i] != p)
            cycle = 1;
    }
}

bool check_res()
{
    int i;

    for (i = 0; i < G.size(); i++)
        if (G[i].size() != 2 || !mark[i])
            return 0;
    return 1;
}

bool check_good()
{
    int i;

    for (i = 0; i < G.size(); i++)
        if (G[i].size() > 2)
            return 0;
    return 1;
}

void add()
{
    int i, j;

    for (i = 0; i < G.size(); i++)
        for (j = i + 1; j < G.size(); j++)
            if (G[i].size() < 2 && G[j].size() < 2)
            {
                mark.assign(G.size(), 0);
                dfs(i);

                if (!mark[j])
                {
                    res.push_back(mp(i, j));
                    G[i].push_back(j);
                    G[j].push_back(i);
                }
            }
}

int main()
{
    int n, m, i, a, b;
    cin >> n >> m;
    G.resize(n);
    mark.resize(n);

    if (n == 1)
    {
        if (m == 0)
            cout << "YES\n1\n1 1\n";
        else if (m == 1)
            cout << "YES\n0\n";
        else
            cout << "NO\n";
        return 0;
    }

    for (i = 0; i < m; i++)
    {
        cin >> a >> b;

        if (a == b)
        {
            cout << "NO" << endl;
            return 0;
        }
        G[a - 1].push_back(b - 1);
        G[b - 1].push_back(a - 1);
    }
    dfs(0);

    if (check_res())
    {
        cout << "YES" << endl;
        cout << 0 << endl;
    }
    else if (check_good() && m <= n && !cycle)
    {
        cout << "YES" << endl;
        add();
        cout << res.size() + 1 << endl;
        a = -1, b = -1;

        for (i = 0; i < res.size(); i++)
            cout << res[i].first + 1 << " " << res[i].second + 1 << endl;

        for (i = 0; i < G.size(); i++)
            if (G[i].size() < 2)
            {
                if (a == -1)
                    a = i;
                else
                    b = i;
            }
        cout << a + 1 << " " << b + 1 << endl;
    }
    else
        cout << "NO" << endl;
    return 0;
}