PKU2553 the bottom of the graph

       昨天skyprophet 神牛给我讲了一句用Tarjan求强连通分量，感觉并不太难，于是试了一下。它只是在基本的DFS中添了一段话。就是对一个顶点，当我们发现他满足low=d 时，它就是一个强连通分量的顶部。我们同时需要用一个栈来维护点序列，通过出栈的方式求强连通。这个方法是线性的，时间效率高于kosaraju算法。而且它跟求桥，求割点的方法是统一的。

program bottom; const maxn=5100; type pointer=^node; node=record data:longint; next:pointer; end; var edge:array[0..maxn] of pointer; low,d,belong,stack,vis:array[0..maxn] of longint; oudg:array[0..maxn] of boolean; n,m,time,top,sum:longint; flag:boolean; function getmin(a,b:longint):longint; begin if a<b then exit(a); exit(b); end; procedure link(a,b:longint); var p:pointer; begin new(p); p^.data:=b; p^.next:=edge[a]; edge[a]:=p; end; procedure prework; var i:longint; begin sum:=0; top:=0; time:=0; fillchar(oudg,sizeof(oudg),0); fillchar(vis,sizeof(vis),0); fillchar(d,sizeof(d),0); for i:=0 to maxn do edge[i]:=nil; end; procedure init; var i,a,b:longint; begin for i:=1 to m do begin read(a,b); link(a,b); end; end; procedure dfs(u:longint); var v:longint; p:pointer; begin inc(vis[u]); inc(top); stack[top]:=u; inc(time); d[u]:=time; low[u]:=time; p:=edge[u]; while p<>nil do begin v:=p^.data; if vis[v]=1 then low[u]:=getmin(low[u],d[v]) else if vis[v]=0 then begin dfs(v); low[u]:=getmin(low[u],low[v]); end; p:=p^.next; end; if low[u]=d[u] then begin inc(sum); repeat v:=stack[top]; dec(top); belong[v]:=sum; inc(vis[v]); until v=u; end; end; procedure main; var i:longint; begin for i:=1 to n do if vis[i]=0 then dfs(i); end; procedure checkgraph; var i,v:longint; p:pointer; begin for i:=1 to n do begin p:=edge[i]; while p<>nil do begin v:=p^.data; if belong[v]<>belong[i] then oudg[belong[i]]:=true; p:=p^.next; end; end; end; procedure outit; var i:longint; begin flag:=false; for i:=1 to n do if not oudg[belong[i]] and not flag then begin flag:=true; write(i); end else if not oudg[belong[i]] and flag then write(' ',i); writeln; end; procedure bigmain; begin while true do begin readln(n,m); if n=0 then exit; prework; init; main; checkgraph; outit; end; end; begin assign(input,'a.in'); reset(input); assign(output,'a.out'); rewrite(output); bigmain; close(input); close(output); end.