1、B站视频链接：E34 树形DP 树的中心_哔哩哔哩_bilibili

#include <bits/stdc++.h> 
using namespace std;
const int N=20010;
int n,a,b,c,ans=2e9;
struct edge{int v,w;};
vector<edge> e[N];
int d1[N],d2[N],path[N],up[N];//path记录d1 

void dfs1(int x,int fa){
	for(auto ed:e[x]){
		int y=ed.v,z=ed.w;
		if(y==fa)continue;
		dfs1(y,x);
		if(d1[y]+z>d1[x]){
			d2[x]=d1[x],d1[x]=d1[y]+z,path[x]=y;
		}else if(d1[y]+z>d2[x])d2[x]=d1[y]+z;
	}
}
void dfs2(int x,int fa){
	for(auto ed:e[x]){
		int y=ed.v,z=ed.w;
		if(y==fa)continue;
		if(y==path[x])up[y]=max(up[x],d2[x])+z;
		else up[y]=max(up[x],d1[x])+z;
		dfs2(y,x);
	}
}
int main(){
	cin>>n;
	for(int i=1;i<n;i++){
		cin>>a>>b>>c;
		e[a].push_back({b,c});
		e[b].push_back({a,c});
	}
	dfs1(1,0);//向下 
	dfs2(1,0);//向上 
	for(int i=1;i<=n;i++){
		ans=min(ans,max(d1[i],up[i]));
	}
	cout<<ans;
	return 0;
}