python/TangDou/Topic/HuanGenDp/CF1406C.cpp

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

// 链式前向星
int e[M], h[N], idx, w[M], ne[M];
void add(int a, int b, int c = 0) {
    e[idx] = b, ne[idx] = h[a], w[idx] = c, h[a] = idx++;
}

int sz[N];  // sz[i]:以i为根的子树中节点个数
int son[N]; // son[i]:去掉节点i后，剩下的连通分量中最大子树节点个数
int r1, r2, n;

void dfs(int u, int fa) {
    sz[u] = 1;  // u为根的子树中，最起码有一个节点u
    son[u] = 0; // 把节点u去掉后，剩下的连通分量中最大子树节点个数现在还不知道，预求最大，先设最小

    for (int i = h[u]; ~i; i = ne[i]) { // 枚举u的每一条出边
        int v = e[i];
        if (v == fa) continue;
        dfs(v, u);
        sz[u] += sz[v];
        son[u] = max(son[u], sz[v]);
    }
    son[u] = max(son[u], n - sz[u]);
    if ((son[u] << 1) <= n) r2 = r1, r1 = u;
}
/*
1 2
1 2
1 3
2 3
*/
signed main() {
#ifndef ONLINE_JUDGE
    freopen("CF1406C.in", "r", stdin);
#endif
    int T;
    cin >> T;
    while (T--) {
        cin >> n;
        // 多组测试数据，清空
        memset(sz, 0, sizeof sz);
        memset(son, 0, sizeof son);
        // 初始化链式前向星
        memset(h, -1, sizeof h);
        idx = 0;

        r1 = r2 = 0;

        for (int i = 1; i < n; i++) { // n-1条边
            int x, y;
            cin >> x >> y;
            add(x, y), add(y, x);
        }

        dfs(1, 0); // 以1号点为入口，它的父节点是0

        if (!r2) {
            int r3 = e[h[r1]];
            cout << r1 << " " << r3 << endl;
            cout << r1 << " " << r3 << endl;
        } else {
            int r3 = r1;

            for (int i = h[r2]; ~i; i = ne[i]) {
                r3 = e[i];
                if (r3 != r1) break;
            }
            cout << r3 << " " << r2 << endl;
            cout << r3 << " " << r1 << endl;
        }
    }
    return 0;
}