PKU 1655 Balancing Act(树+树的重心)

#include<cstdio>
#include<cstring>
#include<algorithm>
#define maxn 20005
using namespace std;
struct Edge
{
    int to;
    int next;
}e[2*maxn];
int t,n,x,y,tot;
int head[maxn],num[maxn],balance[maxn];

void Init()
{
    tot=0;
    memset(head,-1,sizeof(head));
} 
void Addedge(int u,int v)
{
    e[tot].to=v;
    e[tot].next=head[u];
    head[u]=tot++;
}
void dfs(int u,int pre)
{
    balance[u]=0,num[u]=1;
    for(int i=head[u];i!=-1;i=e[i].next){
        int v=e[i].to;
        if(v==pre)
            continue ;
        dfs(v,u);
        balance[u]=max(balance[u],num[v]);
        num[u]+=num[v];
    }
    balance[u]=max(balance[u],n-num[u]);
}
int main()
{
    scanf("%d",&t);
    while(t--){
        scanf("%d",&n);
        Init();
        for(int i=1;i<n;i++){
            scanf("%d%d",&x,&y);
            Addedge(x,y);
            Addedge(y,x);
        }
        dfs(1,-1);
        int pos=1,ans=balance[1];
        for(int i=2;i<=n;i++){
            if(balance[i]<ans){
                pos=i;
                ans=balance[i];
            }
        }
        printf("%d %d\n",pos,ans);
    }
}