### Question:
Tree { public List<Character> postorder() { List<Character> postorder = new ArrayList<>(); postorder(root, postorder::add); return postorder; } Tree(Node root); List<Character> inorder(); List<Character> preorder(); List<Character> postorder();  }### Answer:
@Test public void postorder() { Tree tree = new Tree( new Node( 'F', new Node('B', null, new Node('A')), new Node('E', new Node('C'), new Node('D')))); assertThat(tree.postorder()) .containsExactly('A', 'B', 'C', 'D', 'E', 'F'); }