(* The following code implements stacks (of integers) using linked
lists. *)
type
nodeptr = ^node;
node = record
info: integer;
link: nodeptr
end;
stack = nodeptr;
procedure Create(var s: stack);
(* Makes s an empty stack *)
begin
s := nil
end;
function Isempty(s: stack): boolean;
(* Returns true if s is empty *)
begin
Isempty := s = nil
end;
procedure Push(i: integer; var s: stack);
(* Pushes i onto stack s. *)
var
n: nodeptr; (* Newly added node *)
begin
(* Get a node *)
new(n);
(* Load it up *)
n^.info := i;
(* Link it in *)
n^.link := s;
s := n
end;
{ Stacks p 2 }
procedure Pop(var s: stack);
(* Removes and discards top item from s *)
var
n: nodeptr; (* Node being discarded *)
begin
if s = nil then
(* Error *)
else
begin
n := s;
s := s^.link;
dispose(n)
end
end;
function Top(s: stack): integer;
(* Returns top item on stack s *)
begin
if s = nil then
(* Error *)
else
Top := s^.info
end;