Hint
#include<iostream>#include<cstdio>#include<string.h>#include<algorithm>#include<math.h>#include<stdio.h>#include<ctype.h>#include<queue>using namespace std;struct node{int u,v,next;} edge[200000];struct w{int step,date;} t,f;int cnt,head[200000],vis[200000],p,s;void add(int u,int v){edge[cnt].u=u;edge[cnt].v=v;edge[cnt].next=head[u];head[u]=cnt++;}void bfs(int b){memset(vis,0,sizeof(vis));queue<w>q;t.step=1;t.date=b;vis[b]=1;q.push(t);while(!q.empty()){t=q.front();q.pop();p=t.date;s=t.step;int u=t.date;for(int i=head[u]; i!=-1; i=edge[i].next){f.date=edge[i].v;if(!vis[f.date]){f.step=t.step+1;vis[f.date]=1;q.push(f);}}}}int main(){int n;while(scanf("%d",&n)!=EOF){if(n==1){printf("1\n");continue;}cnt=0;memset(head,-1,sizeof(head));for(int i=1; i<n; i++){int u,v;scanf("%d %d",&u,&v);add(u,v);add(v,u);}bfs(1);bfs(p);printf("%d\n",s);}return 0;}
，人生不能缺乏的是雨夜－－淅沥地，独自的雨夜。这样的雨夜里，
