有向边tarjan

void tarjan(int u){
	dfn[u]=low[u]=++num;
	stack[++top]=u;
	instack[u]=true;
	for(int i=hd[u];i;i=bl[i].nt){
		if(!dfn[bl[i].to]){
			tarjan(bl[i].to);
			low[u]=minn(low[u],low[bl[i].to]);
		}
		else if(instack[bl[i].to])
			low[u]=minn(low[u],dfn[bl[i].to]);
	}
	if(dfn[u]==low[u]){
		int tan=stack[top];
		++tot;
		while(tan!=u){
			belong[tan]=tot;
			instack[tan]=false;
			tan=stack[--top];
		}
		belong[tan]=tot;
		instack[tan]=false;
	}
}