树的直径，几乎板子题，用一遍dfs找寻起始点和长度，两次dfs建立重链，LCA求出起始点最近公共祖先，往上枚举，输出，完事。

注意：十年OI一场空，____________。

重链LCA直通连接

AC 代码

#include<bits/stdc++.h>
#define int long long
using namespace std;
const int N=5e5+10;
vector<pair<int,int> >G[N];
int d[N],d2[N],n,ans,st,ed,s[N],s2[N];
//d表示当前节点往下的最长路径长度,s存储终点
//d2和s2表示次长路径
//ans,st,ed存储全树最长路径及其起始点 
int son[N],siz[N],tp[N],fa[N],dep[N];
void dfs(int x,int xfa,int dis)//找寻直径长度并存储起始点 
{
	s[x]=s2[x]=x;
	for(auto i:G[x])if(i.first!=xfa)
	{ 
		dfs(i.first,x,dis+i.second);
		int t=d[i.first]+i.second;
		if(t>d[x])d2[x]=d[x],s2[x]=s[x],d[x]=t,s[x]=s[i.first];
		else if(t>d2[x])d2[x]=t,s2[x]=s[i.first];//更新 
	}
	if(d[x]+d2[x]>ans)ans=d[x]+d2[x],st=s[x],ed=s2[x];
	if(d[x]+dis>ans)ans=d[x]+dis,st=s[x],ed=1;//不要忘记往上的路径长度 
}
void dfs1(int x,int xfa)//重链LCA 
{
	siz[x]=1;son[x]=-1;fa[x]=xfa;dep[x]=dep[xfa]+1;
	for(auto i:G[x])if(i.first!=xfa)
	{
		dfs1(i.first,x);
		siz[x]+=siz[i.first];
		if(siz[i.first]>siz[son[x]]||son[x]==-1)son[x]=i.first;
	}
}
void dfs2(int x,int top)
{
	tp[x]=top;
	if(son[x]!=-1)dfs2(son[x],top);
	for(auto i:G[x])if(i.first!=fa[x]&&i.first!=son[x])dfs2(i.first,i.first);
}
int LCA(int x,int y)
{
	for(;tp[x]!=tp[y];x=fa[tp[x]])if(dep[tp[x]]<dep[tp[y]])swap(x,y);
	return dep[x]<dep[y]?x:y; 
}
signed main()
{
	scanf("%lld",&n);
	for(int i=1,x,y,w;i<n;i++)
	{
		scanf("%lld%lld%lld",&x,&y,&w);x++,y++;
		G[x].push_back({y,w});
		G[y].push_back({x,w});
	}
	dfs(1,0,0);
	printf("%lld ",ans);
	dfs1(1,0);dfs2(1,1);
	int lca=LCA(st,ed);
	vector<int>up1,up2;//记录答案 
	for(;st!=lca;st=fa[st])up1.push_back(st);
	up1.push_back(lca);//不能忘lca 
	for(;ed!=lca;ed=fa[ed])up2.push_back(ed);
	reverse(up2.begin(),up2.end());//反过来才是答案 
	printf("%lld\n",up1.size()+up2.size());
	for(int i:up1)printf("%lld ",i-1);
	for(int i:up2)printf("%lld ",i-1);//输出 
	return 0;//完结撒花
}

#include<bits/stdc++.h>
using namespace std;
const int N=5e5+10;
#define int long long
#define PII pair<int,int>
#define fi first
#define se second
int mx[N],mx1[N],mx2[N],son1[N],son2[N],rt[N];
vector<PII>G[N];
void dfs(int x,int f)
{
	for(auto i:G[x])if(i.fi!=f)
	{
		int y=i.fi,w=i.se;
		dfs(y,x);
		if(mx1[x]<mx1[y]+w)
			mx2[x]=mx1[x],mx1[x]=mx1[y]+w,son2[x]=son1[x],son1[x]=y;
		else if(mx2[x]<mx1[y]+w)
			mx2[x]=mx1[y]+w,son2[x]=y;
		if(mx[x]<mx[y])
			mx[x]=mx[y],rt[x]=rt[y];
	}
	if(mx[x]<=mx1[x]+mx2[x])
		mx[x]=mx1[x]+mx2[x],rt[x]=x;
}
deque<int>q;
void dfs1(int x)
{
	q.push_back(x);
	if(son1[x])dfs1(son1[x]);
}
void dfs2(int x)
{
	if(son1[x])dfs2(son1[x]);
	q.push_back(x);
}
void output(int x)
{
	cout<<mx[x]<<' ';
	if(son1[x])dfs2(son1[x]);
	q.push_back(x);
	if(son2[x])dfs1(son2[x]);
	cout<<q.size()<<'\n';
	for(int i:q)cout<<i-1<<' ';
}
signed main()
{
	int n;cin>>n;
	for(int i=1;i<n;i++)
	{
		int x,y,w;cin>>x>>y>>w;x++,y++;
		G[x].push_back({y,w});
		G[y].push_back({x,w});
	}
	dfs(1,0);
	output(rt[1]);
	return 0;
}