树的直径

#include<iostream>
#include<cstdio>
#include<cmath>
#include<algorithm>
#include<queue>
#include<cstring>
#define N 10010
using namespace std;
struct node
{
    int u,v,w,nxt;
}e[N*2];
int first[N],cnt;
void ade(int u,int v,int w)
{
    e[++cnt].nxt=first[u]; first[u]=cnt;
    e[cnt].u=u; e[cnt].v=v; e[cnt].w=w;
}
int n,m;
bool vis[N];
int dis[N];
void bfs(int x)
{
    queue<int>q;
    q.push(x);vis[x]=1;
    while(!q.empty())
    {
        int u=q.front(); q.pop();
        for(int i=first[u];i;i=e[i].nxt)
        {
            int v=e[i].v;
            if(vis[v]) continue;
            dis[v]=dis[u]+e[i].w;
            vis[v]=1;
            q.push(v);
        }
    }
}
int maxx,pos;
int main()
{
    scanf("%d%d",&n,&m);
    for(int i=1,x,y,z;i<=m;i++)
    {
        scanf("%d%d%d",&x,&y,&z);
        ade(x,y,z);
    }
    bfs(1);
    for(int i=1;i<=n;i++)
    {
        if(maxx<dis[i])
        {
            maxx=dis[i];
            pos=i;
        }
    }
    //cout<<pos<<" "<<maxx<<endl;
    memset(dis,0,sizeof(dis));
    memset(vis,0,sizeof(vis));
    bfs(pos);
    int maxb=0,posb=0;
    for(int i=1;i<=n;i++)
    {
        if(maxb<dis[i])
        {
            maxb=dis[i];
            posb=i;
        }
    }
    //cout<<posb<<" "<<maxb;
    printf("%d",maxb);
    return 0;
}