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99 LISP problems: Problem #12

Solution to the 99 LISP Problems #12

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(defun decompress-one (n elem)
  (if (<= n 0)
    nil
    (append
      (list elem)
      (decompress-one (1- n) elem))))

(defun decode-rle (encoding)
  (if (null encoding)
    nil
    (append
      (if (listp (car encoding))
        (decompress-one (caar encoding) (second (car encoding)))
        (list (car encoding)))
      (decode-rle (cdr encoding)))))

Lisp dialect: Steel Bank Common Lisp