294. Validate Binary Search Tree

Given the root of a binary tree, determine if it is a valid binary search tree (BST).

A valid BST is defined as follows:

• The left subtree of a node contains only nodes with keys strictly less than the node's key.
• The right subtree of a node contains only nodes with keys strictly greater than the node's key.
• Both the left and right subtrees must also be binary search trees.

Examples

// Tree: [2,1,3]  →  true (valid BST)
// Tree: [5,1,4,null,null,3,6]  →  false (Node 3 in right subtree is less than root 5)