python/TangDou/Topic/HuanGenDp/P3478.cpp

#include <bits/stdc++.h>
using namespace std;
const int N = 1000010, M = N << 1;
#define int long long
#define endl "\n"

int n, id;
// 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 ans;
int f[N], dep[N], size[N];

void dfs1(int u, int fa) {
    size[u] = 1;
    dep[u] = dep[fa] + 1;
    for (int i = h[u]; ~i; i = ne[i]) {
        int v = e[i];
        if (v == fa) continue;
        dfs1(v, u);
        size[u] += size[v];
    }
}
void dfs2(int u, int fa) {
    for (int i = h[u]; ~i; i = ne[i]) {
        int v = e[i];
        if (v == fa) continue;
        f[v] = f[u] + n - 2 * size[v];
        dfs2(v, u);
    }
}
signed main() {
    memset(h, -1, sizeof h);
    cin >> n;
    for (int i = 1; i < n; i++) { // n-1条边
        int a, b;
        cin >> a >> b;
        add(a, b), add(b, a); // 换根DP，无向图
    }
    // 1、第一次dfs,找出：xxx
    dfs1(1, 0);

    // 2、换根
    for (int i = 1; i <= n; i++) f[1] += dep[i];
    dfs2(1, 0);

    // 3、找答案
    for (int i = 1; i <= n; i++)
        if (ans < f[i]) ans = f[i], id = i;
    cout << id << endl;
}