树的重心练习

POJ 1655
模板题
代码：

#include<cstdio>
#include<vector>
#include<algorithm>
using namespace std;

const int INF=0x3f3f3f3f;
const int maxn=20000+100;

vector<int>G[maxn];
int son[maxn];
int  num,ans;
bool vis[maxn];
int n;

inline void DFS(int u){

    int len=G[u].size();
    vis[u]=true;
    son[u]=1;
    int maxson=0;
    for(int i=0;i<len;i++){

        int v=G[u][i];
        if(vis[v]) continue;
        DFS(v);
        son[u]+=son[v];
        maxson=max(maxson,son[v]);
    }
    maxson=max(maxson,n-son[u]);
    if(maxson<num || (maxson==num && u<ans)){

        num=maxson;
        ans=u;
    }
}

int main(){

    int T;
    scanf("%d",&T);
    while(T--){

        scanf("%d",&n);
        for(int i=1;i<=n;i++) G[i].clear(),vis[i]=false;
        for(int i=1;i<n;i++){

            int u,v;
            scanf("%d%d",&u,&v);
            G[u].push_back(v);
            G[v].push_back(u);
        }
        num=ans=INF;
        DFS(1);
        printf("%d %d\n",ans,num);
    }
}

POJ 3107
也是模板题，好像卡用 vector 存图（反正我用 vector T了）
代码：

#include<cstdio>
#include<vector>
#include<algorithm>
using namespace std;

const int INF=0x3f3f3f3f;
const int maxn=50000+100;

struct Edge{

    int to,next;
}edge[maxn<<1];
int head[maxn],edgenum;
int son[maxn];
int  ans[maxn],tot,num;
bool vis[maxn];
int n;

inline void DFS(int u){

    vis[u]=true;
    son[u]=1;
    int maxson=0;
    for(int i=head[u];i!=-1;i=edge[i].next){

        Edge e=edge[i];
        int v=e.to;;
        if(vis[v]) continue;
        DFS(v);
        son[u]+=son[v];
        maxson=max(maxson,son[v]);
    }
    maxson=max(maxson,n-son[u]);
    if(maxson<num){

        num=maxson;
        tot=0;
        ans[tot++]=u;
    }
    else if(maxson==num) ans[tot++]=u;
}

int main(){

    while(scanf("%d",&n)!=EOF){

        for(int i=1;i<=n;i++) vis[i]=false,head[i]=-1;
        edgenum=0;
        for(int i=1;i<n;i++){

            int u,v;
            scanf("%d%d",&u,&v);
            edge[edgenum].to=v;
            edge[edgenum].next=head[u];
            head[u]=edgenum++;
            edge[edgenum].to=u;
            edge[edgenum].next=head[v];
            head[v]=edgenum++;
        }
        num=INF;
        DFS(1);
        sort(ans,ans+tot);
        tot--;
        for(int i=0;i<=tot;i++) printf("%d%c",ans[i],i==tot?'\n':' ');
    }
}