# 99 questions/Solutions/62

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Collect the internal nodes of a binary tree in a list

An internal node of a binary tree has either one or two non-empty successors. Write a predicate internals/2 to collect them in a list.

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

Alternative solution only using cons:

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