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 **less than** the node's key. - The right subtree of a node contains only nodes with keys **greater than** the node's key. - Both the left and right subtrees must also be binary search trees.   **Example 1:** **Input:** root = [2,1,3] **Output:** true ``` **Example 2:** **Input:** root = [5,1,4,null,null,3,6] **Output:** false **Explanation:** The root node's value is 5 but its right child's value is 4. ```   **Constraints:** - The number of nodes in the tree is in the range `[1, 104]`. - `-231 <= Node.val <= 231 - 1`