Danh sách móc nối kép ( không đồ họa)

Program Baitap5;
Uses crt;
Type
     contro=^truong;
     truong=record
           data:string[20];
           Next,Prev :contro;
     end;
Var
  Pdau,Pcuoi:contro;
  chose : char;
Procedure tao_ds;
Var
  dkien:STRING;
  P,R:contro;
Begin
clrscr;
Pdau :=nil;
Repeat
writeln('Bo trong du lieu de thoat.');
write('nhap vao du lieu ');
readln(dkien);
if (dkien <> '') then
   begin
     new(p);
     p^.data:=dkien;
     if Pdau=nil then
        begin
           P^.Next :=nil;
           P^.Prev :=nil;
           Pdau:=P;
           Pcuoi:=P;
        end
      else
        begin
           R:=Pcuoi;
           R^.Next:=P;
           P^.Next :=nil;
           P^.prev :=R;
           Pcuoi:=P;
        end;
      END;
Until dkien='';
end;
Procedure inds;
Var P: contro;
Begin
     clrscr;
     if Pdau =nil then
        write ('khong co du lieu de in ra')
     else
         begin
              writeln('Danh sach du lieu duoc in ra theo chieu thuan');
              P:=Pdau;
              writeln;
              write('Pdau--',#16);
              Repeat
                write(P^.data:4);
                P:=P^.Next;
                write(' --',#16);
              Until P=nil;
              writeln('Nil');
              writeln;
              P:=Pdau;
              write('Nil',#17,'--');

              Repeat
                write(p^.data:4,'  ');
                P:=P^.Next;
                write(#17,'--');
              until P=nil;
              write('Pcuoi');
         End;
         readln;
End;

Procedure bo_sungds;
Var
   data1 :string;
   P,Q,R: contro;
   kt:boolean ;
Begin
clrscr;
   write ('nhap du lieu cua con tro Q ');
   readln(data1); P:=Pdau;
   kt:=false;
   repeat
       if data1 =P^.data then
          begin
             kt:=true;
             new(Q);
             write('nhap vao du lieu bo sung');
             readln(Q^.data);
             if P=Pdau then
                 begin
                     Pdau:=Q;
                     Q^.Next:=P;
                     P^.Prev:=Q;
                 end
             else
                 begin
                     R:=P^.Prev;
                     Q^.Next:=R^.next;
                     R^.next:=Q;
                     Q^.Prev:=P^.Prev;
                     P^.prev:=Q;
                 end;
          end;
       P:=P^.Next;
   until P=nil;
   writeln('**************ket thuc bo sung ************');
   if kt=false then write('khong tim thay du lieu vua nhap');
   readln;
end;
Begin
   Repeat
   CLRSCR;
   writeln;
   writeln('***TUY CHON CHUC NANG ***');
   writeln('1 nhap vao danh sach');
   writeln('2 in ra danh sach theu chieu thuan ');
   writeln('3 bo sung danh sach ');
   writeln('4 save and exit ');
   writeln('Chuc nang ban chon: ');
   readln(chose);
   case chose of '1':tao_ds;
                 '2':inds;
                 '3':bo_sungds;
   end;
   until chose ='4';
end.