POJ 1655 Balancing Act[樹的重心]

#include <cstdio>
#include <cstring>
#include <algorithm>
#include <vector>
using namespace std;
#define N 20010
int s[N], f[N], n, root;
vector<int> g[N];
void getroot(int now, int fa) {
    int u;
    s[now] = 1;
    f[now] = 0;
    for (int i=0; i<g[now].size(); i++)
        if ((u=g[now][i]) != fa) {
            getroot(u, now);
            s[now] += s[u];
            f[now] = max(f[now], s[u]);
        }
    f[now] = max(f[now], n-s[now]);
    if (f[now] < f[root]) root = now;
}
int main() {
#ifndef ONLINE_JUDGE
    freopen("in.txt", "r", stdin);
#endif
    int T, a, b;
    scanf("%d", &T);
    while (T--) {
        scanf("%d", &n);
        for (int i=1; i<=n; i++) g[i].clear();
        for (int i=1; i<n; i++) {
            scanf("%d%d", &a, &b);
            g[a].push_back(b); g[b].push_back(a);
        }
        f[0] = n;
        getroot(1, root=0);
        int cnt = 0;
        for (int i=1; i<=n; i++) if (f[i] == f[root]) {
            root = i; break;
        }
        printf("%d %d\n", root, f[root]);
    }

    return 0;
}