981C - Useful Decomposition - CodeForces Solution

#include <bits/stdc++.h>
using namespace std;
#define ull unsigned long long
#define ll long long
#define ld long double
#define fr first
#define sec second
#define mem(dp, x) memset(dp, x, sizeof dp)
#define loop(a, b, c) for (ll(a) = (b); a < (c); a++)
#define cin(v)        \
    for (auto &c : v) \
    cin >> c
#define cout(v)           \
    for (auto &c : v)     \
        cout << c << " "; \
    cout << endl;
#define coutpair(v)  \
    for (auto c : v) \
        cout << c.fr << sp << c.sec << endl;
#define cingrid(grid, n, m)        \
    for (ll i = 0; i < n; ++i)     \
        for (ll j = 0; j < m; ++j) \
            cin >> grid[i][j];
#define coutgrid (grid, n, m) for (ll i = 0; i < n; ++i) for (ll j = 0; j < m; ++j) cout << grid[i][j] << " ";
#define all(v) v.begin(), v.end()
#define nl cout << "\n"
#define yes cout << "Yes" << endl
#define no cout << "No" << endl
#define sp " "
#define tt    \
    ll t;     \
    cin >> t; \
    while (t--)
#define pb push_back
void fast()
{
    cout << setprecision(18);
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
}
const int N = 1e5+5;
vector<int> adj[N];
int main()
{
    int n;
    cin >> n;
set<int>st;
    loop(i,1,n)
    {
        int u, v;
        cin >> u >> v;
       st.insert(u);
st.insert(v);
        adj[u].pb(v);
        adj[v].pb(u);
    }
    int maxx = 0,no_3=0;
    vector<int> nodes;

    for(auto i:st)
{
    if(adj[i].size()>=3)
       maxx=i, no_3++;
        if (adj[i].size() == 1)
            nodes.pb(i);
}

if(no_3>=2)
        no;
else if(nodes.size()==2) 
{
        yes;
        cout << 1 << endl;
        cout << nodes[0] << " " << nodes[1] << endl;
}
else 
{
        yes;
        cout << nodes.size() << endl;
        for(auto c:nodes)
            cout << maxx << " " << c << endl;
}
}