Stumped with functional breadth-first tree traversal in Clojure?

Question

Say I have a tree defined as per the recommendation in this post, although it\'s a vector in my case, which hopefully shouldn\'t matter (they\'re vectors in Programming Clojure

Answer 1

Your tree data is incorrect. It should be [1 [2 [4] [5]] [3 [6]]]

Also, you're mixing the tree traversal with printing and building up a result. Things get simpler if you concentrate on doing the hard part separately:

(def tree [1 [2 [4] [5]] [3 [6]]])

NOTE THIS IS DEPTH-FIRST. SEE BELOW

(defn bf "return elements in tree, breath-first"
   [[el left right]] ;; a tree is a seq of one element,
                     ;; followed by left and right child trees
   (if el
     (concat [el] (bf left) (bf right))))

(bf tree)
=> (1 2 4 5 3 6)

CORRECT VERSION

(defn bf [& roots] 
   (if (seq roots) 
       (concat (map first roots) ;; values in roots
               (apply bf (mapcat rest roots))))) ;; recursively for children

(bf tree)
=> (1 2 3 4 5 6)