Just because I apparently can't think in an interview room.

longestsubstring.clj

(defn findSubstring
  "Finds the longest substring"
  [s]
  (->> (range (dec (count s)) 0 -1)
       (map #(->> s
                    (partition % 1)
                    (map (partial apply str))
                    (getSubstring)))
       (drop-while nil?)
       first))

(defn getSubstring
  "returns the string if substring match otherwise nil"
  [candidates]
  (->> candidates
       (frequencies)
       (filter #(< 1 (second %)))
       (ffirst)
       )
  )