Find the set of characters two strings have in common

chars-in-common.clj

;; Solve the problem "find the set of characters two strings have in common"

;; Approach #1 BitSets
;; - use a bitsets for each string,
;; - flip the bit for each character in each string
;; - then "and" the two bitsets. Theoretically O(N + M) performance

(defn string-to-charbits [s]
  (loop [[f & rest] s
          bits (java.util.BitSet.)]
    (if f (recur rest (do (.set bits (int f)) bits))
           bits)))

(defn process-bitset [bitset]
   (loop [bitset bitset
          count 0
          result '[]]
     (if (< count (.length bitset))
       (recur bitset
              (inc count)
              (if (.get bitset count) (conj result (char count)) result))
       result)))

(defn find-matching-chars [s1 s2]
  (let [b1 (string-to-charbits s1)
        b2 (string-to-charbits s2)]
    (do
      (.and b1 b2)
      (process-bitset b1))))

;; Approach #2 - Brute Force
;; For each character in string 1, linear search (via .indexOf) of string 2
;; Theoretically O(N * M) performance

(defn find-matching-chars-brute [s1 s2]
  (loop [[f & rest] s1
         s2 s2
         result []]
    (if f
      (if (> (.indexOf s2 (int f)) -1)
        (recur rest s2 (conj result f))
        (recur rest s2 result))
      (distinct result))))

;; Might need to sort the results of each function call to make this pass
(assert (= (find-matching-chars "abcd" "cde")
            (find-matching-chars-brute "abcd" "cde"))
         "expected [\\c \\d]")

Submitted for code review:
http://codereview.stackexchange.com/questions/13521/find-the-common-characters-between-two-strings-without-using-set-operations