Validate Binary Search Tree

Write `is_valid_bst(root)` returning `True` when every node in the left subtree is strictly less than the node, and every node in the right subtree is strictly greater — for *all* nodes, not just immediate children. [5,1,4,null,null,3,6] -> False (3 sits right of 5) Comparing each node against its parent is the classic wrong answer: it accepts a tree where a deep descendant violates an ancestor's bound. Pass a (low, high) range down instead, narrowing it at each step.