# 99 questions/Solutions/71

From HaskellWiki

(*) Determine the internal path length of a tree.

We define the internal path length of a multiway tree as the total sum of the path lengths from the root to all nodes of the tree. By this definition, `tree5` has an internal path length of 9.

```
ipl :: Tree a -> Int
ipl = ipl' 0
where ipl' d (Node _ ts) = d + sum (map (ipl' (d+1)) ts)
```