CF #328div2 D

这题不难,当时想出来了,可是却写不出来~~

现在慢慢写回来,也写得好挫~

可以知道,被攻击的城市必定可以组成一棵树,然后,传送到的点必定也是城市之一。如果出发后回到原点,则需要2E,E是树的边数,则2E-D就是答案,D是树中直径。

我写得好锉~

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#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <queue>
#define LL long long
using namespace std;
 
const int MAX=123460;
 
struct Edge{
    int u,v,next;
}edge[MAX*2];
 
int head[MAX],dis[MAX];
int n,m,tot;
bool chose[MAX];
int pre[MAX],point[MAX],deg[MAX];
int dis1[MAX],node1[MAX],dis2[MAX],node2[MAX];
bool vis[MAX];
 
void addedge(int u,int v){
    edge[tot].u=u;
    edge[tot].v=v;
    edge[tot].next=head[u];
    head[u]=tot++;
}
 
int BFS(int rt,int counts){
    queue<int>que;
    que.push(rt);
    vis[rt]=true;
    int u,v;
    while(!que.empty()&&counts){
        u=que.front();
        que.pop();
        vis[u]=true;
        if(chose[u]) counts--;
        for(int e=head[u];e!=-1;e=edge[e].next){
            v=edge[e].v;
            if(vis[v]) continue;
            que.push(v);
            pre[v]=u;
        }
    }
    memset(vis,false,sizeof(vis));
    while(!que.empty()) que.pop();
     
     
    for(int i=1;i<=m;i++) que.push(point[i]);
    while(!que.empty()){
        u=que.front();
        que.pop();
        v=pre[u];
        if(v!=-1){
            if(!vis[v]){
                que.push(v);
                chose[v]=false;
            }
            vis[v]=true;
        }
    }
     
    while(!que.empty()) que.pop();
    memset(vis,false,sizeof(vis));
    for(int i=1;i<=m;i++) if(chose[point[i]]) que.push(point[i]);
    memset(deg,0,sizeof(deg));
    int ans=0;
    while(!que.empty()){
        u=que.front();
        vis[u]=true;
        que.pop();
        v=pre[u];
        if(v!=-1){
            ans++;
            if(!vis[v])
                que.push(v);
            deg[v]++;
            vis[v]=true;
        }
    }
     
    while(!que.empty()) que.pop();
    for(int i=1;i<=m;i++) if(chose[point[i]]) que.push(point[i]);
    while(!que.empty()){
        u=que.front();
        dis[dis1[u]]=dis[dis1[u]]==0?min(node1[u],u):min(min(node1[u],u),dis[dis1[u]]);
        dis[dis1[u]+dis2[u]]=dis[dis1[u]+dis2[u]]==0?min(node1[u],node2[u]):min(min(node1[u],node2[u]),dis[dis1[u]+dis2[u]]);
        que.pop();
        v=pre[u];
        if(v!=-1){
            deg[v]--;
            if(!deg[v]) que.push(v);
            dis1[u]+=1;
            if(dis1[u]>dis1[v]){
                dis2[v]=dis1[v],node2[v]=node1[v];
                dis1[v]=dis1[u],node1[v]=node1[u];
            }
            else if(dis1[u]==dis1[v]){
                if(node1[u]<node1[v]){
                    dis2[v]=dis1[v],node2[v]=node1[v];
                    node1[v]=node1[u];
                }
                else{
                    if(dis1[u]>dis2[v]){
                        dis2[v]=dis1[u],node2[v]=node1[u];
                    }
                    else if(dis1[u]==dis2[v]){
                        if(node1[u]<node2[v]){
                            node2[v]=node1[u];
                        }
                    }
                }
            }
            else{
                if(dis1[u]>dis2[v]){
                    dis2[v]=dis1[u],node2[v]=node1[u];
                }
                else if(dis1[u]==dis2[v]){
                    if(node1[u]<node2[v]){
                        node2[v]=node1[u];
                    }
                }
            }
             
        }
    }
/// cout<<ans<<endl;
/// printf("%d\n",node1[rt]);
    for(int i=n;i>=1;i--){
        if(dis[i]>0){
            printf("%d\n",dis[i]);
            return ans*2-i;
        }
    }
    printf("%d\n",point[1]);
    return ans;
     
}
 
int main(){
    int u,v;
    while(scanf("%d%d",&n,&m)!=EOF){
        for(int i=1;i<=n;i++){
            head[i]=-1,chose[i]=false,pre[i]=-1;
            dis1[i]=dis2[i]=0,node1[i]=node2[i]=i;
            vis[i]=false; dis[i]=0;
        }
        tot=0;
        for(int i=1;i<n;i++){
            scanf("%d%d",&u,&v);
            addedge(u,v);
            addedge(v,u);
        }
        for(int i=1;i<=m;i++){
            scanf("%d",&u);
            point[i]=u;
            chose[u]=true;
        }
        printf("%d\n",BFS(u,m));
         
         
    }
    return 0;
}

  

posted @   chenjunjie1994  阅读(140)  评论(0编辑  收藏  举报
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