树上dp求直径的模板
#include<iostream>
#include<cstdio>
#include<cstring>
#include<vector>
using namespace std;
int n;
vector<int> v[100001];
int x,y;
int f1[100001];
int f2[100001];
int ans;
void dfs(int x,int f){
for(int i=0;i<v[x].size();++i){
if(v[x][i]==f) continue;
dfs(v[x][i],x);
int tem=f1[v[x][i]]+1;
if(tem>f1[x]){
f2[x]=f1[x];
f1[x]=tem;
}else if(tem>f2[x]){
f2[x]=tem;
}
}
ans=max(ans,f2[x]+f1[x]);
}
int main(){
scanf("%d",&n);
for(int i=1;i<n;++i){
scanf("%d%d",&x,&y);
v[x].push_back(y);
v[y].push_back(x);
}
dfs(1,0);
cout<<ans;
return 0;
}