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. The call stack holds function calls up to the deepest leaf node.
inOrder(node) {if (node === null) return;
inOrder(node.left); // Traverse left subtree
visit(node); // Process current node
inOrder(node.right); // Traverse right subtree
}