求树的直径

void bfs(int u)
{
    queue<int> Q;
    mem(d, 0);
    mem(vis, 0);
    Q.push(u);
    d[u] = 0;
    vis[u] = 1;
    maxway = 0, pos = u;
    while(!Q.empty())
    {
        u = Q.front(); Q.pop();
        for(int i=head[u]; i != -1; i=Edge2[i].next)
        {
            int v = Edge2[i].v;
            if(vis[v]) continue;
            vis[v] = 1;
            d[v] = d[u] + 1;
            if(d[v] > maxway)
                maxway = d[v], pos = v;
            Q.push(v);
        }
    }
}

int main()
{
    bfs(1);
    bfs(pos);
    return 0;
}