警示后人：如果你 RE 90 on test 7

void dfs(int u){
	son[u] = -1, siz[u] = 1;
	int len = g[u].size();
	for(int i = 0; i < len; i ++){
		int v = g[u][i];
		if(!dep[v]){
			g0[u].push_back(v);//
			dep[v] = dep[u] + 1;
			fa[v] = u;
			dfs(v);
			siz[u] += siz[v];
			if(son[u] == -1 || siz[v] > siz[son[u]]) son[u] = v;
		}
		else if(v != fa[u]) edges[++ edgecnt].first = u, edges[edgecnt].second = v;//
	}
	return ;
}
void dfs(int u, int t){
	top[u] = t, dfn[u] = ++ cnt, rnk[cnt] = u;
	if(son[u] == -1) return ;
	dfs(son[u], t);
	int len = g0[u].size();
	for(int i = 0; i < len; i ++){
		int v = g0[u][i];
		if(v != son[u] && v != fa[u]) dfs(v, v);
	}
	return ;
}