Validate a Binary Search Tree

Write pseudocode or code to determine whether a binary tree is a valid Binary Search Tree (BST). A valid BST requires every value in a node's left subtree to be strictly smaller than the node, and every value in its right subtree to be strictly larger. The input is a level-order array representation of a binary tree, where null represents a missing node. Output true or false. Examples Input: [2,1,3] Output: true Input: [5,1,4,null,null,3,6] Output: false Node values may be any 32-bit integers. Comparing a node only with its direct children is insufficient. Example Input 2 1 3 Output true