I was given following task in the interview:

Given a string, find shortest substring in it that contains all of the different characters contained in the original string.

Here is my solution in emacs lisp:

(defun add-to-hash (c ht)
  "Add an instance of character C to hash table HT"
  (puthash c (1+ (gethash c ht 0)) ht))

(defun remove-from-hash (c ht &optional try)
  "Remove an instance of character C from hash table HT
Return nil if no instances of C remain in HT"
  (let ((old (gethash c ht 0)))
    (if (and try (< 2 old))
      (if (> 1 (puthash c (1- (gethash c ht 0)) ht))
          (remhash c ht)

(defun find-min-substring (sstr)
  "Find minimal substring that contains all the characters in a given string"
  ;; get all characters
  (let* ((all-chars (make-hash-table))
         (slen (length sstr))
         (fcnt (progn
                 (mapc (lambda (c) (add-to-hash c all-chars)) sstr)
                 (hash-table-count all-chars)))
         (beg 0) (end fcnt)
         (res sstr))
    (if (= end slen)
      (let* ((cand (substring sstr beg end))
             (cand-chars (make-hash-table))
             (ccnt (progn
                     (mapc (lambda (c) (add-to-hash c cand-chars)) cand)
                     (hash-table-count cand-chars))))
        ;; find first candidate, that is a substring with all the characters
        (while (< ccnt fcnt)
          (add-to-hash (aref sstr end) cand-chars)
          (setq end (1+ end))
          (setq ccnt (hash-table-count cand-chars)))
        (setq cand (substring sstr beg end))
        ;; shorten it as much as possible
        (while (remove-from-hash (aref sstr beg) cand-chars t)
          (setq beg (1+ beg)))
        (setq cand (substring sstr beg end))
        (setq res cand)
        ;; check other variants
        (while (< end slen)
          ;; advance both ends
          (add-to-hash (aref sstr end) cand-chars)
          (setq end (1+ end))
          (remove-from-hash (aref sstr beg) cand-chars)
          (setq beg (1+ beg))
          (setq ccnt (hash-table-count cand-chars))
          ;; another candidate is found
          (when (= ccnt fcnt)
            ;; only change candidate if it was shortened
            (while (remove-from-hash (aref sstr beg) cand-chars t)
              (setq beg (1+ beg))
              (setq cand (substring sstr beg end)))
            (setq res cand)))

1 Answer 1


I know this is old, but I will write some remarks anyways, hope you will find it useful.

  1. This code lacks knowledge of usual language shortcuts, eg. (if x nil y) is equivalent to (unless x y). Similarly, (setq x (1+ x)) is the same as (incf x), (setq x y) (setq z q) is the same as (setq x y z q) inside (implicit) progn. (puthash c (1- (gethash c ht 0)) ht) is the same as (decf (gethash c ht 0)).
  2. Language offers specialized data-structure for working with characters--char-table.
  3. It is better to express algorithmic code in terms it is easy for humans to interpret (i.e. leave the low-level details of implementation to helper functions, while giving helpers meaningful names. For example add-to-hash isn't particularly better than puthash. Perhaps better partitioning of your code would involve functions like unique-characters-of, generate-candidate etc.

Finally, and most importantly, your code doesn't really solve the problem, try this for instance:

(find-min-substring "aaaaabacbaaaaac") ; => "acbaaaa"

Below is an alternative solution. It is very straight-forward (probably inefficient), but (hopefully) correct:

(defun string-cost (s)
   with mask = (make-vector 128 0)
   for c across s do
   (setf (aref mask c) 1)
   finally (cl-return (cl-reduce '+ mask))))

(defun generate-candidates (s)
  (list (substring s 0 (1- (length s)))
        (substring s 1)))

(defun choose-candidate (a b min-cost)
  (let ((a-candidate (min-common-substring a min-cost))
        (b-candidate (min-common-substring b min-cost)))
    (if (< (length a-candidate) (length b-candidate))

(defun min-common-substring (s &optional min-cost)
  (unless min-cost (setf min-cost (string-cost s)))
  (cl-destructuring-bind (a b)
      (generate-candidates s)
    (let ((a-cost (string-cost a))
          (b-cost (string-cost b)))
       ((and (>= a-cost min-cost)
             (>= b-cost min-cost))
        (choose-candidate a b min-cost))
       ((>= a-cost min-cost)
        (min-common-substring a min-cost))
       ((>= b-cost min-cost)
        (min-common-substring b min-cost))
       (t s)))))

Where more involved, but slightly more efficient version of string-cost could look like this:

(defun string-cost (s)
   with mask = (make-char-table t 0)
   for c across s do
   (set-char-table-range mask c 1)
    (let ((cost 0))
       #'(lambda (k v)
           (when (and (/= v 0) (consp k))
             (setf v (- (cdr k) (1- (car k)))))
           (incf cost v))
  • \$\begingroup\$ Does the 128 in your code mean that your function only works for ASCII? \$\endgroup\$ Oct 15, 2017 at 18:27
  • \$\begingroup\$ @RolandIllig yup, that was the intention, but also see the second string-cost where it uses char-table which can adjust its size based on character range used. \$\endgroup\$
    – wvxvw
    Oct 16, 2017 at 6:04
  • \$\begingroup\$ Thanks for answering! It seems error in my code was where i used "<" instead of ">", and shortening of strings failed to occur. Did not test it through... Will educate myself on things you have mentioned, starting with cl-loop. \$\endgroup\$
    – Srv19
    Oct 16, 2017 at 14:45

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