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))
        nil
      (if (> 1 (puthash c (1- (gethash c ht 0)) ht))
          (remhash c ht)
        t))))
 
(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)
        res
      (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)))
        res))))