Solve https://www.facebook.com/photo.php?fbid=1868600983366318

trees.clj

(ns a-namespace
  (:require [clojure.string :refer [join]]))

(defn dashes []
  (->> (iterate dec 10)
       (map #(repeat % "-"))
       (map join)))

(defn draw [f]
  (->> (map vector (dashes) (f))
       (take 10)
       (map join)
       (map println)))

(defn xes []
  (->> (iterate #(+ 2 %) 1)
       (map #(repeat % "x"))
       (map join)))

(defn gen-inc [c]
  (cons (inc (count c)) c))

(defn palindroms []
  (map #(str (join %1) (join %2))
       (map reverse (iterate gen-inc '(1)))
       (iterate gen-inc '())))

(defn some-reverse []
  (let [normal (->> (iterate #(+ 4 %) 3)
                    (map #(take % (iterate inc 1)))
                    (map join))
        reverse (->> (iterate #(+ 4 %) 1)
                     (map #(take % (iterate inc 1)))
                     (map reverse)
                     (map join))]
    (interleave reverse normal)))

(draw xes)
(draw palindroms)
(draw some-reverse)