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

 1 const maxe=50001;
 2 type
 3   node=record
 4     f,t:longint;
 5   end;
 6 var n,m,dgr,i,u,v,num,ans:longint;
 7 bfsdgr,low,head,f:array[0..maxe] of longint;
 8 b:array[0..maxe] of node;
 9 p:array[0..maxe] of boolean;
10 procedure insert(u,v:longint);
11 begin
12   with b[i] do
13     begin
14       f:=head[u];
15       t:=v;
16     end;
17   head[u]:=i;
18 end;
19 procedure tarjan(x:longint);
20 var e,v:longint;
21 begin
22   inc(dgr);
23   e:=head[x]; if e=0 then exit;
24   inc(num);
25   bfsdgr[x]:=dgr; low[x]:=dgr;
26   f[num]:=x; p[x]:=true;
27   //
28   while e<>0 do
29     begin
30       v:=b[e].t;
31       if bfsdgr[v]=0 then
32         begin
33           tarjan(v);
34           if low[v]<low[x] then low[x]:=low[v];
35         end
36       else if (p[v]) and (bfsdgr[v]<low[x]) then
37         low[x]:=bfsdgr[v];
38       e:=b[e].f;
39     end;
40   if bfsdgr[x]=low[x] then
41     begin
42       inc(ans);
43       //writeln(x);
44       //repeat
45       while f[num]<>x do
46         begin
47           p[f[num]]:=false;
48           dec(num);
49         end;
50       //until f[num]=x;
51     end;
52 end;
53 begin
54   readln(n,m);
55   for i:=1 to m do
56     begin
57       readln(u,v);
58       insert(u,v);
59     end;
60   fillchar(bfsdgr,sizeof(bfsdgr),0);
61   fillchar(p,sizeof(p),false);
62   for i:=1 to n do
63     if bfsdgr[i]=0 then tarjan(i);
64   writeln(ans);
65 end.