P1654: [Usaco2006 Jan]The Cow Prom 奶牛舞会

裸的强连通

 const maxe=;
 type
   node=record
     f,t:longint;
   end;
 var n,m,dgr,i,u,v,num,ans:longint;
 bfsdgr,low,head,f:array[..maxe] of longint;
 b:array[..maxe] of node;
 p:array[..maxe] of boolean;
 procedure insert(u,v:longint);
 begin
   with b[i] do
     begin
       f:=head[u];
       t:=v;
     end;
   head[u]:=i;
 end;
 procedure tarjan(x:longint);
 var e,v:longint;
 begin
   inc(dgr);
   e:=head[x]; if e= then exit;
   inc(num);
   bfsdgr[x]:=dgr; low[x]:=dgr;
   f[num]:=x; p[x]:=true;
   //
   while e<> do
     begin
       v:=b[e].t;
       if bfsdgr[v]= then
         begin
           tarjan(v);
           if low[v]<low[x] then low[x]:=low[v];
         end
       else if (p[v]) and (bfsdgr[v]<low[x]) then
         low[x]:=bfsdgr[v];
       e:=b[e].f;
     end;
   if bfsdgr[x]=low[x] then
     begin
       inc(ans);
       //writeln(x);
       //repeat
       while f[num]<>x do
         begin
           p[f[num]]:=false;
           dec(num);
         end;
       //until f[num]=x;
     end;
 end;
 begin
   readln(n,m);
   for i:= to m do
     begin
       readln(u,v);
       insert(u,v);
     end;
   fillchar(bfsdgr,sizeof(bfsdgr),);
   fillchar(p,sizeof(p),false);
   for i:= to n do
     if bfsdgr[i]= then tarjan(i);
   writeln(ans);
 end.

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