时间复杂度:O(N)

void dp(int x)
{
    v[x] = 1;
    for(int i = head[x]; i ; i = next[i])
        {
            int y = ver[i];
            if(v[y]) continue;
            dp(y);
            ans = max(ans,d[x] + d[y] + edge[i]);
            d[x] = max(d[x],d[y] + edge[i]);
        }
}