find_pairs_sum_equal_k.clj

(defn find-pairs-sum-equal-k [xs k]
  (let [x-indices (into {} (map-indexed (fn [idx x] [x idx]) xs))]
    (keep (fn [[x idx]]
            (if-let [x-complement-idx (get x-indices (- k x))]
              (when (not= idx x-complement-idx)
                [x (- k x)])))
          x-indices)))

(find-pairs-sum-equal-k (range 0 20) 12)
;; => ([0 12] [7 5] [1 11] [4 8] [3 9] [12 0] [2 10] [11 1] [9 3] [5 7] [10 2] [8 4])