80分的程序：

#include <bits/stdc++.h>
using namespace std;
const int N=1e6+10;
vector<pair<int,int>> G[N];
int n,d1[N],d2[N];

void dfs(int x,int fa) 
{
	for(auto i:G[x]) 
	{
		int y=i.first,c=i.second;if(y==fa)continue;
		dfs(y,x);
		if(d1[y]+c>d1[x])      d2[x]=d1[x],d1[x]=d1[y]+c;
		else if(d1[y]+c>d2[x] )d2[x]=d1[y]+c;
	}
}

int main() 
{
	scanf("%d",&n);
	for(int i=1,x,y,c; i<=n-1; i++) 
	{
		scanf("%d%d%d",&x,&y,&c);
		G[x].push_back({y,c});
		G[y].push_back({x,c});
	}
	int ans=1e9+10;
	for(int i=1;i<=n;i++)
	{
		memset(d1,0,sizeof(d1));memset(d2,0,sizeof(d2));
		dfs(i,0);
		ans=min(ans,d1[i]);
	}	
	printf("%d\n",ans);
	return 0;
}

100分的程序：

#include <bits/stdc++.h>
using namespace std;
const int N=1e6+10;
vector<pair<int,int>> G[N];
int n,d1[N],d2[N],path[N],up[N];

void dfs(int x,int fa) 
{
	for(auto i:G[x]) 
	{
		int y=i.first,c=i.second;if(y==fa)continue;
		dfs(y,x);
		if(d1[y]+c>d1[x]) d2[x]=d1[x],d1[x]=d1[y]+c,path[x]=y;
		else if(d1[y]+c>d2[x] )d2[x]=d1[y]+c;
	}
}
void dfs2(int x,int fa)
{
	for(auto i:G[x]) 
	{
		int y=i.first,c=i.second;if(y==fa)continue;
		if(y==path[x])up[y]=max(up[x],d2[x])+c;
		else          up[y]=max(up[x],d1[x])+c;
		dfs2(y, x);
	}
}
int main() 
{
	scanf("%d",&n);
	for(int i=1,x,y,c; i<=n-1; i++) 
	{
		scanf("%d%d%d",&x,&y,&c);
		G[x].push_back({y,c});
		G[y].push_back({x,c});
	}
	memset(d1,0,sizeof(d1));memset(d2,0,sizeof(d2));
	memset(up,0,sizeof(up));
	dfs(1,0);
	dfs2(1,0);
	int ans=1e9+10;
	for(int i=1; i<=n; i++)ans=min(ans,max(d1[i],up[i]));
	printf("%d\n",ans);
	return 0;
}