(defn sum3
 "Solve the 3SUM problem O(n^2) time."
 [s]
 (def tab @{})
 (def solutions @{})
 (def len (length s))
 (for [k 0 len]
  (put tab (get s k) k))
 (for [i 0 len]
  (for [j 0 len]
   (def k (get tab (- 0 (get s i) (get s j))))
   (when (and k (not= k i) (not= k j) (not= i j))
    (put solutions {i true j true k true} true))))
 (iter2array (map (fn [x] (iter2array (keys x))) (keys solutions))))