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Classic traversal problem
Given the root of a binary tree, return the inorder traversal of its nodes' values.
Example: For tree with root 1, right child 2, and 2's left child 3, the inorder traversal is . This problem seems trivial with recursion.
The real challenge is doing it iteratively, which interviewers often ask as a follow-up. Constraints: up to nodes, values from to .
