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Postorder Traversal

A "postorder" traversal also visits all the nodes in a tree. It's called "postorder" because the current node is visited after its children. The following tree:

Would be visited in this order:

[2, 3, 7, 9, 12, 10, 8, 5]

Assignment

Our data team didn't like the way we ordered the users in our BST export (personally I think they just want to kick the work for themselves down the road). Anyhow, we've been asked to change it.

Implement the recursive postorder method. Here are the algorithm's steps:

Recursively traverse the left subtree
Recursively traverse the right subtree
Visit the value of the current node by appending its value to the visited array
Return array of visited nodes