A list of length 0 or 1 is already sorted. Otherwise, break the list in half, sort the halves, and merge them.

(defun llmergesort (lst)
  (let (mid half)
    (if (or (null lst) (null (rest lst)))
        lst
        (progn (setq mid (midpoint lst))
               (setq half (rest mid))
               (setf (rest mid) nil)
               (dmerj (llmergesort lst)
                      (llmergesort half)) ) )) ))          

public static Cons llmergesort (Cons lst) {
  if ( lst == null || rest(lst) == null)
     return lst;
   else { Cons mid = midpoint(lst);
          Cons half = rest(mid);
          setrest(mid, null);
          return dmerj( llmergesort(lst),
                        llmergesort(half)); } }

What is O()? Stack depth? Conses?

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