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00 Longest Alternating Substring.md

Longest Alternating Substring

Write a function that takes a string of digits and returns the longest substring that contains only alternating odd/even digits. If two substrings have the same length, return the one that appears first.

Examples

(longest-alt-subs "") ;=> ""
(longest-alt-subs "1") ;=> "1"
(longest-alt-subs "123") ;=> "123"
(longest-alt-subs "13") ;=> "1"
(longest-alt-subs "122381") ;=> "2381"

Thanks to this site for the problem idea, where it is rated Expert in Python. The problem has been modified.

Please submit your solutions as comments on this gist.

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(defn longest-alt-subs [s]
  (if (empty? s)
    ""
    (reduce (fn [a b] (max-key count b a))
            (subs s 0 1)
            (re-seq #"(?:[02468][13579])+[02468]?|(?:[13579][02468])+[13579]?" s))))

(require '[clojure.string :as str])

(defn alternating-substrings [s]
  (if (empty? s)
    [s]
    (->> (map vector s (range))
         (partition-by (fn [[char index]]
                         ;; since index alternates we can check if char follows index
                         ;; this helps us to avoid checking alternation of neighbour chars
                         (= (even? (int char)) (even? index))))
         (map #(str/join (map first %))))))

(defn first-longest [xs]
  (apply (partial max-key count) (reverse xs)))

(defn longest-alt-subs [s]
  (first-longest (alternating-substrings s)))

(defn longest-alt-subs [s]
  (->> (range (count s))
       (partition-by #(even? (+ % (int (nth s %)))))
       (map #(subs s (first %) (inc (last %))))
       reverse
       (apply max-key count "")))

(defn longest-alt-subs [s]
  (let [chodd? (fn [ch] (case ch (\1 \3 \5 \7 \9) true false))]
    (if (< (count s) 2)
      s
      (->> (reduce (fn [res c]
                     (let [p (peek (peek res))]
                       (if (= (chodd? p) (chodd? c))
                         (conj res [c])
                         (conj (pop res) (conj (peek res) c)))))
                   [[(first s)]]
                   s)
           rseq
           (apply max-key count)
           (apply str)))))

Somewhat faster to access chars by index in S. Refactored.

(defn longest-alt-subs [s]
  (let [iodd? (fn [i] (case (.charAt ^String s ^int i) (\1 \3 \5 \7 \9) true false))
        len (.length ^String s)]
    (if (< len 2)
      s
      (let [[longstart longend start]
            (reduce (fn [lls i]
                      (if (not= (iodd? (dec i)) (iodd? i))
                        lls
                        (let [[longstart longend start] lls]
                          (if (> (- i start) (- longend longstart))
                            [start i i]
                            [longstart longend i]))))
                   [-1 -1 0]
                   (range 1 len))]
        (if (> (- len start) (- longend longstart))
          (subs s start)
          (subs s longstart longend))))))

(defn longest-alt-subs [s]
  (let [even-odd #"([02468][13579])*[02468][13579]?"
        odd-even #"([13579][02468])*[13579][02468]?"
        empty-str #""
        pattern  (re-pattern (str even-odd "|" odd-even "|" empty-str))
        alt-subs (->> (re-seq pattern s)
                      (map first))]
    (reduce (fn [longest el]
              (if (< (count longest) (count el))
                el
                longest))
     ""
     alt-subs)))

(defn longest-alt-subs [s]
  (->> (take-while seq (iterate rest s))
       (mapcat #(reductions conj [] %))
       (filter #(apply = 1 (map count (partition-by odd? (map int %)))))
       reverse
       (apply max-key count "")
       (apply str)))

EDIT
@mchampine, nice use of rest and reductions to generate windows!
I shamelessly copied your idea - interestingly this is the third of recent challenges
in which a windows function is handy:

(defn windows 
  "Constructs a lazy seq of all contiguous subsequences of xs."
  [xs]
  (->> xs (iterate rest)
       (sequence (comp (take-while seq)
                       (mapcat #(reductions conj [] %))
                       (map seq)
                       (filter identity)))
       (cons ()) lazy-seq))

(defn alternating-subs? [xs]
  (= (count xs) 
     (->> xs (sequence (comp (map int) 
                             (partition-by odd?)))
          count)))

(defn longest-alt-subs [digs]
  (->> digs windows
       (filter alternating-subs?)
       (sort-by (comp - count))
       first
       (apply str)))

(defn char->long [c]
  (- (long c) 48))

(defn longest-alternating-substring [s]
  (let [{:keys [max-alts alts]} (reduce (fn [{:keys [max-alts alts] :as acc} [prev-char next-char]]
                                          (if (or (and (odd? prev-char)
                                                       (even? next-char))
                                                  (and (even? prev-char)
                                                       (odd? next-char)))
                                            (update acc :alts (fnil conj [prev-char]) next-char)
                                            (-> acc
                                                (update :max-alts #(if (> (count alts) (count %))
                                                                     alts
                                                                     %))
                                                (assoc :alts nil))))
                                        {}
                                        (partition 2 1 (map char->long s)))]
    (clojure.string/join "" (if (> (count alts) (count max-alts))
                              alts
                              max-alts))))

loop version

(defn longest-alternating-substring-loop [s]
  (loop [alts nil
         max-alts nil
         [[prev-char next-char :as item] & rest'] (partition 2 1 (map char->long s))]
    (if-not item
      (clojure.string/join "" (if (> (count alts) (count max-alts))
                                alts
                                max-alts))
      (recur (when (or (and (odd? prev-char)
                            (even? next-char))
                       (and (even? prev-char)
                            (odd? next-char)))
               (if (nil? alts)
                 [prev-char next-char]
                 (conj alts next-char)))
             (if (> (count alts) (count max-alts))
               alts
               max-alts)
             rest'))))