Validate Binary Search Tree

Given a binary tree, print true if it's a valid BST (every node's value is strictly greater than every value in its left subtree and strictly less than every value in its right subtree), false otherwise.

Example

Input: 5 1 4 null null 3 6 Output: false (5's right subtree contains 3, which is smaller than 5)

Approach

The classic trap: checking only "left child < node < right child" locally is not sufficient — a node deep in the left subtree could still violate the BST property relative to an ancestor several levels up. Instead, carry a valid (low, high) range down through the recursion, tightening it at every step.

Input format

Line 1: the tree in level-order, space-separated, null for missing children

Print true or false.