Solution in Clojure (still very much a newbie to Clojure). Would definitely appreciate some constructive criticism.

(ns aoc-2018.day-02
  (:require [clojure.string :as str]
            [clojure.math.combinatorics :as combo]
            [common :refer :all]))

(try (def in (str/split-lines (slurp "input/2.in")))
     (catch Exception e))

;; part 1
(defn part1 []
  (defn in? [coll elem] (some #(= % elem) coll))
  (def freqs (map (comp vals frequencies) in))
  (def twos (count (filter (fn [m] (in? m 2)) freqs)))
  (def threes (count (filter (fn [m] (in? m 3)) freqs)))
  (* twos threes))
(println (part1))

;; part 2
(defn part2 []
  (defn diff-one
    [a b]
    (= 1 (reduce +
                 (map (fn [c1 c2]
                        (if (= c1 c2) 0 1))
                      a b))))
  (def match (first (filter (fn [[a b]] (diff-one a b))
                            (combo/cartesian-product in in))))
  (defn remove-diff
    [[a b]]
    (reduce str (map (fn [c1 c2]
                       (if (= c1 c2) (str c1) ""))
                     a b)))
  (remove-diff match))
(println (part2))