python/TangDou/Topic/HuanGenDp/CF1406.cpp

#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 10;
#define int long long
#define endl "\n"

vector<int> g[N];
int siz[N], son[N];
int r1, r2, n;
void dfs(int u, int fa) {
    siz[u] = 1;
    son[u] = 0;
    for (int i = 0; i < g[u].size(); i++) {
        int v = g[u][i];
        if (v == fa) continue;
        dfs(v, u);
        siz[u] += siz[v];
        son[u] = max(son[u], siz[v]);
    }
    son[u] = max(son[u], n - siz[u]);
    if ((son[u] << 1) <= n) r2 = r1, r1 = u;
}
signed main() {
    int t;
    cin >> t;
    while (t--) {
        cin >> n;
        for (int i = 0; i <= n + 10; i++) g[i].clear(), siz[i] = 0, son[i] = 0;
        for (int i = 1; i < n; i++) {
            int x, y;
            cin >> x >> y;
            g[x].push_back(y);
            g[y].push_back(x);
        }
        r1 = r2 = 0;
        dfs(1, 0);
        if (!r2) {
            int r3 = g[r1][0];
            cout << r1 << " " << r3 << endl;
            cout << r1 << " " << r3 << endl;
        } else {
            int r3 = r1;
            for (int i = 0; i < g[r2].size(); i++) {
                r3 = g[r2][i];
                if (r3 != r1) break;
            }
            cout << r3 << " " << r2 << endl;
            cout << r3 << " " << r1 << endl;
        }
    }
    return 0;
}