#1050 : 树中的最长路(两次BFS)

#include <cstdio>
#include <cstring>
#include <queue>
#define MAXN 100010
#define mem(a,b) memset(a,b,sizeof(a))
using namespace std;
int head[MAXN],tol;
struct Edge{
int v,next;
}e[MAXN*2];
void addEdge(int u,int v){
e[tol].v=v;e[tol].next=head[u];head[u]=tol++;
e[tol].v=u;e[tol].next=head[v];head[v]=tol++;
}
struct node{
int u,dis;
node(int u,int dis):u(u),dis(dis){}
};
queue<node> q;
bool vis[MAXN];
int n;
int bfs(int u,int& ans){
while(!q.empty()) q.pop();
q.push(node(u,0));
vis[u]=true;
while(!q.empty()){
node tmp=q.front();q.pop();
u=tmp.u;
ans=tmp.dis;
int v;
for(int k=head[u];k!=-1;k=e[k].next){
v=e[k].v;
if(!vis[v]){
vis[v]=true;
q.push(node(v,tmp.dis+1));
}
}
}
return u;
}
int main(){
scanf("%d",&n);
tol=0;mem(head,-1);
int u,v;
for(int i=1;i<n;++i){scanf("%d%d",&u,&v);addEdge(u,v);}
int ans;
mem(vis,false);
u=bfs(1,ans);
mem(vis,false);
bfs(u,ans);
printf("%d\n",ans);
return 0;
}