> LZ怎么写的这么长啊。。我的多好看！
> const b:array[false..true]of string=
>   (' not','');
> var s:set of 0..100;
>   flag,null:boolean;
>   count,i,x,y:longint;
>   rd,f:array[0..100]of longint;
> function find(x:longint):longint;
>   begin
>     if f[x]=x then exit(x);
>     find:=find(f[x]);
>     f[x]:=find;
>   end;
> begin
>   count:=0;
>   while true do begin
>     s:=[]; flag:=true; null:=false;
>     fillchar(rd,sizeof(rd),0);
>     for i:=1 to 100 do f[i]:=i;
>     while true do begin
>       read(x,y);
>       if x+y=0 then break;
>       if x+y<0 then halt;
>       if x=y then flag:=false;
>       if find(x)=find(y) then flag:=false;
>       f[find(x)]:=find(y);
>       inc(rd[y]); s:=s+[x]+[y];
>     end;
>     for i:=1 to 100 do
>       if i in s then begin
>         if rd[i]=0 then begin
>           if null then begin
>             flag:=false; break;
>           end else null:=true;
>         end else begin
>           if rd[i]<>1 then begin
>             flag:=false; break;
>           end;
>         end;
>       end;
>     if s=[] then begin flag:=true; null:=true; end;
>     inc(count);
>     writeln('Case ',count,' is',b[flag and null],' a tree.');
>   end;
> end.