Visualize recursive call stacks, queue progressions, and constant space threaded Morris algorithms.
Every node in the tree is visited exactly once.
H is the height of the tree. Space is required for recursion stack frames representing parents waiting to be visited.
postOrder(node) {if (node === null) return;
postOrder(node.left); // Traverse left subtree
postOrder(node.right); // Traverse right subtree
visit(node); // Process current node
}