SOURCE FILE: inorderNonRecursive

A NON-RECURSIVE IMPLEMENTATION OF AN IN-ORDER TRAVERSAL.

This example illustrates that sometimes a non-recursive method
of solving a problem can be quite complex.

   (* p is a pointer to the tree to be traversed *)
PROCEDURE Inorder(p: NodePtr) ;
VAR S: stack ;  (* declare stack variable S *)
    ok, finished :boolean ;
BEGIN (* procedure Inorder *)
  Stacks.Create(S,ok) ;  (* initialize the stack *)
  WHILE p <> nil  (* as long as p points somewhere *)
  DO BEGIN (* outer while loop *)
          (* as long as you can go left from where p points. *)
       WHILE p^.left <> nil
       DO BEGIN (* inner while loop *)
            Push(S,p) ; (* push p onto the stack *)
            p := p^.left ;   (* advance p to the left *)
          end  (* inner while loop *)
       finished := false ;
       REPEAT
         Process(p) ; (* Print what's in the node, or whatever. *)
         IF p^.right = nil ; (* if there is no node to the right *)
         THEN BEGIN
                IF    NOT Stacks.Empty(S) (* if the stack is not empty *)
                THEN  Pop(S,p) (* pop a pointer off the stack *)
                ELSE  BEGIN
                        finished := true ; (* signal to stop repeat loop *)
                        p := nil ; (* signal to stop outer while loop *)
                      end
              end 
          ELSE BEGIN (* work on the right subtree *)
                 p := p^.right ; (* point at the right subtree *)
                 finished := true ; (* signal to stop repeat loop *)
               END
       UNTIL finished ;
     END (* outer while loop *)
     Stack.Terminate(S) ; (* deallocate the stack *)
END   (* procedure Inorder *)