solution.clj

(defn is-merge [s part1 part2]
  (let [s-len (count s)
        p1-len (count part1)
        p2-len (count part2)]
    (if (> s-len (+ p1-len p2-len))
      false
      (loop [s-idx 0
             p1-idx 0
             p2-idx 0]
        (if (= s-idx s-len)
          true
          (if (and (< p1-idx p1-len) (= (nth s s-idx) (nth part1 p1-idx)))
            (recur (inc s-idx) (inc p1-idx) p2-idx)
            (if (and (< p2-idx p2-len) (= (nth s s-idx) (nth part2 p2-idx)))
              (recur (inc s-idx) p1-idx (inc p2-idx))
              false)))))))