* postOrderTraversal(node = this.root) {
    if (node.left) { yield* this.postOrderTraversal(node.left); }
    if (node.right) { yield* this.postOrderTraversal(node.right); }
    yield node;
  }
Post-order traversal on a tree: left-right-root.
@param {BinaryTreeNode} node first node to start the traversal
@yields {BinaryTreeNode}