Given the root of a binary tree, return the in-order traversal of its node values (Left → Root → Right).
The recursive solution is the cleanest: traverse the left subtree, visit the current node, then traverse the right subtree. An iterative version uses an explicit stack to mimic the call frames.
Example: root = [1,null,2,3] → [1,3,2]. root = [] → [].
[1,3,2]