树的重心（模板）

#include<bits/stdc++.h>using namespace std;const int N = 20005;const int INF = 1<<30;int head[N],son[N],cnt,n,ans,size;bool vis[N];struct Edge {    int to,next;} edge[2*N];void Init() {    cnt = 0;    size = INF;    memset(vis,0,sizeof(vis));    memset(head,-1,sizeof(head));}void add(int u,int v) {    edge[cnt].to = v;    edge[cnt].next = head[u];    head[u] = cnt++;}void dfs(int cur) {    vis[cur] = 1;    son[cur] = 0;    int tmp = 0;    for(int i=head[cur]; i>=0; i=edge[i].next) {        int u = edge[i].to;        if(!vis[u]) {            dfs(u);            son[cur] += son[u] + 1;            tmp = max(tmp,son[u] + 1);        }    }    tmp = max(tmp,n-son[cur]-1);    if(tmp < size || (tmp == size && cur < ans)) {        ans = cur;        size = tmp;    }}int main() {    int T;    scanf("%d",&T);    while(T--) {        Init();        scanf("%d",&n);        for(int i=1; i<=n-1; i++) {            int u,v;            scanf("%d%d",&u,&v);            add(u,v);            add(v,u);        }        dfs(1);        printf("%d %d\n",ans,size);    }    return 0;}