2019牛客暑期多校训练营（第四场）A.meeting 树的直径

#include<bits/stdc++.h>
const int maxn=1e5+10;
using namespace std;
struct node{
    int x,d;
};
vector<int>mp[maxn];
int ty[maxn],vis[maxn],p[maxn];
int ans1=0,ans2=0;
int point1=0,point2=0;
void bfs(int x){
    queue<node>q;
    q.push({x,0});
    vis[x]=1;
    while(!q.empty()){
//        cout<<"qwe"<<endl;
        node k=q.front();q.pop();
        if(ty[k.x]){
            if(k.d>ans1){
                ans1=k.d;point1=k.x;
            }
//                ans1=max(ans1,k.d);
        }
//        cout<<k.x<<endl;
        for(int i=0;i<mp[k.x].size();i++){
            int j=mp[k.x][i];
            if(vis[j])continue;vis[j]=1;
            q.push({j,k.d+1});
        }
    }
    memset(vis,0,sizeof(vis));
    q.push({point1,0});vis[point1]=1;
    while(!q.empty()){
        node k=q.front();q.pop();
        if(ty[k.x]){
            if(k.d>ans2){
                ans2=k.d;point2=k.x;
            }
        }
        for(int i=0;i<mp[k.x].size();i++){
            int j=mp[k.x][i];
            if(vis[j])continue;vis[j]=1;
            q.push({j,k.d+1});
        }
    }
}
int main(){
    int n,k;scanf("%d%d",&n,&k);
    for(int i=1;i<n;i++){
        int u,v;
        scanf("%d%d",&u,&v);
        mp[u].push_back(v);
        mp[v].push_back(u);
    }
    for(int i=1;i<=k;i++){
        scanf("%d",&p[i]);
        ty[p[i]]=1;
    }
//    cout<<"qwe"<<endl;
    bfs(p[1]);
//    cout<<point1<<endl;
//    bfs(point1)
//    cout<<point1<<" "<<point2<<endl;
    int ans=ans2/2;
    if(ans2%2)ans++;
    printf("%d\n",ans);
}