Tarjan

//求强连通分量 
 void uni(int x,int y){
      if (rank[x]<rank[y]){
          fa[x]=y;
          size[y]+=size[x];
    }else{
        rank[x]+=(rank[x]==rank[y]);
        fa[y]=x;
        size[x]+=size[y];
    }
  }

  int getfa(int po){
      int t=po;
      while (t!=fa[t]) t=fa[t];
      
      int root=t;
      while (po!=fa[po]){
        t=fa[po];
      fa[po]=root;
      po=t;    
    }
    
    return(root);
  }

  void tarjan(int po){
      low[po]=dfn[po]=++cnt;
      bt[po]=1;
      ins[po]=1;
      
      sta[++top]=po;
      
      if (bt[des[po]]==0){
        tarjan(des[po]);
        low[po]=min(low[po],low[des[po]]);
    }else{
      if (ins[des[po]]) low[po]=min(low[po],dfn[des[po]]);    
    }
    
    if (low[po]==dfn[po]){
        while (sta[top]!=po){
        ins[sta[top]]=0;
        int t1=sta[top--];
        if (getfa(t1)!=getfa(po)) uni(getfa(t1),getfa(po));
    }
    ins[sta[top--]]=0;
    }
  }