Độ phức tạp $O(n)$ nhé
var a,b: array[1..1000] of longint;
d: array[-10000..10000] of byte;
i,m,n: word;
begin
readln(n,m);
for i:=1 to n do begin read(a[i]); inc(d[a[i]]); end;
writeln;
for i:=1 to m do begin read(a[i]); inc(d[a[i]]); end;
writeln('Cac so la: ');
for i:=1 to n do if d[a[i]] = 1 then write(a[i],' ');
readln
end.