Understanding the stateful postorder traversal pattern

The stateless postorder traversal can only share data between nodes by passing a copy of processed values up from the child nodes to the parent node. However, there are some problems where, to process a node, we may also need to read or update some state variables that can be accessed from all nodes. The stateful postorder traversal is ideal for solving such problems, as the same copy data is shared between all nodes throughout the traversal, which can be read or updated when returned values from the left and right subtrees are aggregated in a node.