Problem - Binary Tree Inorder

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 $[1, 3, 2]$. This problem seems trivial with recursion.

The real challenge is doing it iteratively, which interviewers often ask as a follow-up. Constraints: up to $100$ nodes, values from $-100$ to $100$.