type
PElem = ^Elem;
Elem = record
Prev,Next: PElem;
Data: integer;
end;
var
L, P0, N: PElem;
function AddBefore(D:integer;var P0:PElem):PElem;
var R:PElem;
begin
New(R);
R^.Data:=D; R^.Next:=P0; R^.Prev:=P0^.Prev; P0^.Prev:=R;
if not (R^.Prev=nil) then R^.Prev^.Next:=R;
AddBefore:=R;
end;
begin
randomize;
New(L);
L^.Data:=10+random(100); L^.Prev:=nil; L^.Next:=nil;
New(L^.Next);
P0:=L^.Next;
P0^.Data:=10+random(100); P0^.Prev:=L; P0^.Next:=nil;
Writeln;
Writeln('L : ', L^.Data);
Writeln('L^.Next = P0 : ', L^.Next^.Data);
Writeln('P0 : ', P0^.Data);
Writeln('Add');
N:=AddBefore(random(10), P0);
Writeln('L : ', L^.Data);
Writeln('L^.Next = N : ', L^.Next^.Data);
Writeln('L^.Next^.Next = P0 : ', L^.Next^.Next^.Data);
Writeln('P0 : ', P0^.Data);
Writeln('N : ', N^.Data);
end.
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