支配树

(code:)

void dfs_dfn(int x)
{
	dfn[x]=++dfn_cnt;
	id[dfn_cnt]=x;
	for(int i=a.head[x];i;i=a.e[i].nxt)
	{
		int y=a.e[i].to;
		if(dfn[y]) continue;
		fa[y]=x;
		dfs_dfn(y);
	}
}
int find(int x)
{
	if(x==fath[x]) return x;
	int tmp=find(fath[x]);
	if(dfn[sdom[val[fath[x]]]]<dfn[sdom[val[x]]])
		val[x]=val[fath[x]];
	return fath[x]=tmp;
}
void tarjan()
{
	for(int i=1;i<=n;++i) sdom[i]=fath[i]=val[i]=i;
	for(int i=dfn_cnt;i>1;--i)
	{
		int x=id[i];
		for(int i=b.head[x];i;i=b.e[i].nxt)
		{
			int y=b.e[i].to;
			if(!dfn[y]) continue;
			find(y);
			if(dfn[sdom[val[y]]]<dfn[sdom[x]])
				sdom[x]=sdom[val[y]];
		}
		c.add(sdom[x],x);
		fath[x]=fa[x];
		x=fa[x];
		for(int i=c.head[x];i;i=c.e[i].nxt)
		{
			int y=c.e[i].to;
			find(y);
			if(sdom[val[y]]==x) idom[y]=x;
			else idom[y]=val[y];
		}
		c.head[x]=0;
	}
	for(int i=2;i<=dfn_cnt;++i)
	{
		int x=id[i];
		if(idom[x]!=sdom[x])
			idom[x]=idom[idom[x]];
	}
	for(int i=2;i<=n;++i) d.add(idom[i],i);
}