【POJ 2631】 Roads in the North

#include<iostream>
#include<algorithm>
#include<queue>
#include<cmath>
#include<cstring>
#include<cstdlib>
#include<cstdio>
using namespace std;
const int N=20005;
const int oo=0x3f3f3f3f;
int x,y,z;
struct node{
    int v,l,next;
}e[N];
int vis[N],d[N],head[N];
int ans,k,cnt,biu;
void add_bian(int u,int v,int l){
    cnt++; e[cnt].v=v; e[cnt].l=l;
    e[cnt].next=head[u]; head[u]=cnt;
    
    cnt++; e[cnt].v=u; e[cnt].l=l;
    e[cnt].next=head[v]; head[v]=cnt;
}

void dfs(int u,int t){
    for(int i=head[u];i;i=e[i].next){
        int v=e[i].v;
        if(vis[v]==0){
            vis[v]=1;
            d[v]=t+e[i].l;
            if(d[v]>ans) 
               { ans=d[v]; biu=v; }
            dfs(v,d[v]);
        }
    }
}

int main(){
    freopen("2631.in","r",stdin);
    freopen("2631.out","w",stdout);
    while(scanf("%d %d %d",&x,&y,&z)!=EOF)
        add_bian(x,y,z);
        
    memset(vis,0,sizeof(vis));
    vis[1]=1; ans=0; dfs(1,0);
    
    memset(vis,0,sizeof(vis));
    vis[biu]=1; ans=0; dfs(biu,0);
    
    printf("%d\n",ans);
    return 0;
}