rt，有人能帮我改对的话我用三个号给他关注，Code:

#include<iostream>
#include<algorithm>
#define int long long
#define rep for(int i=head[u];i!=-1;i=e[i].next)
#define clear(a) for(int i=0;i<N;++i)a[i]=0;
using namespace std;

const int N=1e6+5;

int mark;

int n,m,p,l,r,ans,cnt;
int head[N],dep[N],d[N],dist[N];
int jump[N][25];

struct edge{
	int v,w,next;
}e[N];

inline void add(int u,int v,int w){
	e[++cnt].v=v;
	e[cnt].w=w;
	e[cnt].next=head[u];
	head[u]=cnt;
	return ;
}

inline void dfsA(int u,int f){
	if(d[u]>d[p])p=u;
	rep{
		int v=e[i].v,w=e[i].w;
		if(v==f)continue;
		d[v]=d[u]+w;
		dfsA(v,u);
	}
	return ;
}

inline void dfsB(int u,int f){
	rep{
		int v=e[i].v;
		if(v==f)continue;
		dep[v]=dep[u]+1;
		jump[v][0]=u;
		for(int i=1;i<=20;++i){
			jump[v][i]=jump[jump[v][i-1]][i-1];
		}
		dfsB(v,u);
	}
	return ;
}

inline void recPath(){
	dfsA(1,0);
	r=p;
	d[r]=0;
	dfsA(r,0);
	l=p;
	return ;
}

inline void recEdge(int u,int f){
	rep{
		int v=e[i].v;
		if(v==f)continue;
		dist[v]=dist[u]+e[i].w;
		recEdge(v,u);
	}
	return ;
}

inline int LCA(int x,int y){
	if(dep[x]<dep[y])swap(x,y);
	for(int i=20;i>=0;--i){
		if(dep[jump[x][i]]>=dep[y]){
			x=jump[x][i];
		}
		if(dep[x]==dep[y])break;
	}
	for(int i=20;i>=0;--i){
		if(jump[x][i]!=jump[y][i]){
			x=jump[x][i];
			y=jump[y][i];
		}else{
			break;
		}
	}
	return x==y?x:jump[x][0];
}

inline int dis(int x,int y){
	return dist[x]+dist[y]-dist[LCA(x,y)]*2;
}

signed main(){
//	freopen("in.txt","r",stdin);
	for(int i=1;i<N;++i)head[i]=-1;
	cin>>n>>m;
	for(int i=1;i<=n-1;++i){
		int u,v,w;
		cin>>u>>v>>w;
		add(u,v,w);
		add(v,u,w);
	}
	recPath();
	dfsB(1,0);
//	cout<<"done\n";
	recEdge(1,0);
	int b=l,c=r;
	for(int a=1;a<=n;++a){
		ans=max(ans,min(dis(a,b),dis(a,c)));
	}
	cout<<ans+dis(b,c)<<"\n";
	return 0;
}
//Wu_Ruiheng 4980pts rk526
//Lywh_ddAC  3331pts rk1234