This question evaluates understanding of binary tree traversal, comparing recursive and iterative (explicit stack) implementations and their time and space complexity. Commonly asked in Coding & Algorithms interviews to assess both practical implementation and conceptual understanding of tree data structures, algorithmic trade-offs, and complexity analysis.

Given the root of a binary tree, return the in-order ordering of its node values. Implement both a recursive solution and an iterative solution using an explicit stack, and analyze their time and space complexity.