> 一遍0msAC！！！
> 这么ws的模拟竟然一遍AC激动啊
> 代码：
> program poj1308;
> const
>   maxn=100;
> 
> var
>   b,bj,dd:array[1..maxn] of longint;
>   x,y,t,s,d,bb:longint;
> 
> function fa(x:longint):boolean;
> begin
>   if dd[x]=1 then exit(false);
>   if b[x]<>0 then
>   begin
>     dd[x]:=1;
>     fa:=fa(b[x]);
>   end
>   else exit(true);
> end;
> 
> begin
>   read(x,y); t:=0;
>   while (x=0) and (y=0) do
>   begin
>     inc(t);
>     writeln('Case ',t,' is a tree.');
>     read(x,y);
>   end;
>   while (x<>-1) and (y<>-1) do
>   begin
>     inc(t);
>     fillchar(b,sizeof(b),0);
>     fillchar(bj,sizeof(bj),0);
>     s:=0; d:=0; bb:=0;
>     while (x<>0) and (y<>0) do
>     begin
>       inc(s);
>       if bj[x]=0 then
>       begin
>         bj[x]:=1; inc(d);
>       end;
>       if bj[y]=0 then
>       begin
>         bj[y]:=1; inc(d);
>       end;
> 
>       if b[y]=0 then b[y]:=x
>       else
>       begin
>         bb:=1;
>       end;
> 
>        fillchar(dd,sizeof(dd),0);
>       if not(fa(y)) then
>       begin
>         bb:=1;
>       end;
>       read(x,y);
>     end;
>     if s+1<>d then bb:=1;
>     if bb=1 then
>       writeln('Case ',t,' is not a tree.')
>     else
>       writeln('Case ',t,' is a tree.');
>     read(x,y);
>     while (x=0) and (y=0) do
>     begin
>       inc(t);
>       writeln('Case ',t,' is a tree.');
>       read(x,y);
>     end;
>   end;
> end.