void tarjan(int u)

 {

     dfn[u]=low[u]=++dfs_clock;

     stack_push(u);

     for (int c=head[u];c;c=nxt[c])

     {

         int v=to[c];

         if (!dfn[v])

         {

             tarjan(v);

             low[u]=min(low[u],low[v]);

         }

         else if (!scc[v])

             low[u]=min(low[u],dfn[v]);

     }

     if (low[u]==dfn[u])

     {

         scc_cnt++;

         int cur;

         for (;;)

         {

             scc_size[scc_cnt]++;

             cur=stack_pop();

             scc[cur]=scc_cnt;

             if (cur==u)

                 break;

         }

     }

 }