Python: Binary trees have three common traversals: preorder, inorder, and postorder. Your task is to complete the methods preorder, inorder, and postordernote that each of these methods returns a list, but doesnt print anything. def preorder(self) -> list: """Return a list of this tree's items using a *preorder* traversal. """ def inorder(self) -> list: """Return a list of this tree's items using an *inorder* traversal. """ def postorder(self) -> list: """Return a list of this tree's items using a *postorder* traversal. """