Clojure BFS

bfs.clj

(defn BFS [tree]
  (loop [roots [tree]
         ret   []]
    (if (empty? roots)
      ret
      (let [children (->> roots
                          (mapcat :children)
                          (remove nil?))
            this-level (map :val roots)]
        (recur children
               (concat ret this-level))))))

;; test
;; ====
(def tree
  {:val 1
   :children [{:val 2}
              {:val 3
               :children [{:val 4
                           :children [{:val 6}]}
                          {:val 5}]}]})

(BFS tree)
;; => (1 2 3 4 5 6)