Trees: in-order traversal

An in-order traversal visits a binary tree in the order: left subtree, then this node, then right subtree. The magic trick: do this on a binary search tree and the values come out sorted. (That's not a coincidence — it's the whole point of a BST.)

Walk build_bst([5, 3, 8, 2, 4, 7]) left → node → right and the values fall out already sorted:

        5
       / \
      3   8
     / \  /
    2  4 7      visit order: 2 → 3 → 4 → 5 → 7 → 8   (sorted!)

The trees here are built by build_bst(values), which inserts each int in order, sending values < node left and >= right.

Your task

Write inorder(root) returning a list of the values in in-order (left, node, right). For a BST that list is sorted ascending.

inorder(build_bst([5, 3, 8, 2, 4, 7]))   # [2, 3, 4, 5, 7, 8]
inorder(build_bst([]))                    # []
inorder(build_bst([3, 1, 2]))            # [1, 2, 3]

Hint: an empty node yields []; otherwise glue together inorder(left) + [root.val] + inorder(right).