# Problem 68b

Write a function to traverse a tree and return the values in a list in in-order.

## Example

``````tree67 =
Node 'a'
(Node 'b'
(Node 'd' Empty Empty)
(Node 'e' Empty Empty))
(Node 'c'
Empty
(Node 'f'
(Node 'g' Empty Empty)
Empty))

inorder tree67 == ['d', 'b', 'e', 'a', 'c', 'g', 'f']
``````

## Unit Test

``````import Html
import List

type Tree a
= Empty
| Node a (Tree a) (Tree a)

inorder : Tree a -> List a
inorder tree =
-- your implementation goes here
[]

main =
Html.text
(if (test) then
"Your implementation passed all tests."
else
"Your implementation failed at least one test."
)

test : Bool
test =
List.all ((==) True)
[ inorder Empty == []
, inorder tree == (List.range 1 7)
, inorder tree67 == ['d', 'b', 'e', 'a', 'c', 'g', 'f']
, inorder tree68 == [3, 2, 4, 1, 5, 7, 9, 8, 6]
]

tree67 =
Node 'a'
(Node 'b'
(Node 'd' Empty Empty)
(Node 'e' Empty Empty))
(Node 'c'
Empty
(Node 'f'
(Node 'g' Empty Empty)
Empty))

tree68 =
Node 1
(Node 2
(Node 3 Empty Empty)
(Node 4 Empty Empty))
(Node 5
Empty
(Node 6
(Node 7
Empty
(Node 8
(Node 9 Empty Empty)
Empty))
Empty))

tree =
Node 6
(Node 2
(Node 1 Empty Empty)
(Node 4
(Node 3 Empty Empty)
(Node 5 Empty Empty)))
(Node 7 Empty Empty)
``````

Solution