99 Clojure Problems – 61: Count the Leaves of a Binary Tree.

"A leaf is a node with no successors. Write a method leaf-count to count them."

(deftest p61-count-leaves
  (is  (= (leaf-count '[x [x [x nil nil] nil] [x nil nil]]) 2)))

I simply walked the tree and counted all the leaves. Both functions, for detecting leaves and walking the tree, existed from previous exercises. See the solution on Github.

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