transforming trees in lisp

Question

I\'m trying to modify a representation of a tree from : (A 2 B 0 C 2 D 0 E 0) in (A (B) (C (D) (E))). My code is like:

(defun transform(l)
 (cond
   ( (null l)          


        

Answer 1

An answer to this is given by https://stackoverflow.com/a/34193414/1250772 as part of a response to a user who is evidently working through the same homework problem. The solution is based on reversing the prefix notation into postfix, and then interpreting it as a stack-based reverse polish notation for constructing a tree.

By coincidence, the following code from that answer produces the same representation as what you're asking for. I came up with that representation impromptu, in order to solve the inorder traversal problem in that question:

(defun build-tree (syntax)
  (let ((rs (reverse syntax))
        (stack))
    (dolist (item rs (pop stack))  ;; heart of the interpreter loop
      (cond ((integerp item) (push item stack))  ;; integer instruction
            ((symbolp item) (let ((num (pop stack)))  ;; sym instruction
                              ;; construct node using backquote, and
                              ;; put it on the stack.
                              (push `(,item ,@(loop repeat num
                                                    collect (pop stack)))
                                    stack)))))))