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Validate a Binary Search Tree is a medium coding interview question covering trees, binary search, recursion — the kind asked at FAANG (Facebook/Meta, Apple, Amazon, Netflix, Google) and AI-first companies like Anthropic, OpenAI, and Nvidia.
Given the root of a binary tree, determine whether it is a valid binary search tree. Every node must be greater than all values in its left subtree and less than all values in its right subtree. Note that [5,4,6,null,null,3,7] is NOT valid: 3 sits under 6 and is smaller than it, but it is in 5's right subtree.
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