Pick the +/- operator to put between each digit from 1 to N

zero-sum-game.clj

(defn zero-sum-game [n]
  (let [explode
         (fn explode [old item]
           (concat (map #(cons    item  %) old)
                   (map #(cons (- item) %) old)))
        candidate-combinations
          (->> (range 2 (inc n))
               (reduce explode ['(1)]))
        sum-is-zero?
          (fn sum-is-zero? [a-seq-of-numbers] (zero? (apply + a-seq-of-numbers)))
        zero-combinations
          (filter sum-is-zero? candidate-combinations)
        num->string
          #(if (< 1 %) (str \+ %) (str %))
        seq->string
          (fn seq->string [a-seq]
            (->> a-seq
                 (map num->string)
                 reverse
                 (apply str)
                 #_(interpose " ")
                 (apply str)))
        nice-strings
          (map seq->string zero-combinations)]
     nice-strings))

(zero-sum-game 7)