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VAR
s,s1,s2:string;
le,po:integer;
procedure search(s1,s2:string);
var
l,p:integer;
begin
l:=length(s1);
p:=pos(s1[1],s2);
if copy(s1,1,p-1)<>'' then search(copy(s1,2,p-1),copy(s2,1,p-1));
if copy(s1,p+1,l-p)<>'' then search(copy(s1,p+1,l-p),copy(s2,p+1,l-p));
write(s2[p]);
end;
BEGIN
while not eof do
begin
read(s);
le:=length (s);
po:=pos(' ',s);
s1:=copy(s,1,po-1); s2:=copy(s,po+1,po-1);
search(s1,s2);
end;
END.

知道前序中序求后序