99 questions/Solutions/61A

Collect the leaves of a binary tree in a list

A leaf is a node with no successors. Write a predicate leaves/2 to collect them in a list.

leaves :: Tree a -> [a]
leaves  Empty                 = []
leaves (Branch a Empty Empty) = [a]
leaves (Branch a left  right) = leaves left ++ leaves right

Alternative solution only using cons:

leaves t = leaves' t []
    where leaves'  Empty                 xs = xs
          leaves' (Branch x Empty Empty) xs = x:xs
          leaves' (Branch _ l r)         xs = leaves' l $ leaves' r xs