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.
Constraints
The number of nodes in the tree is in the range [1, 10^4]
-2^31 <= Node.val <= 2^31 - 1
Examples
Input: root = [2,1,3]
Output: true
Left child 1 < root 2 < right child 3. Valid BST.
Input: root = [5,1,4,null,null,3,6]
Output: false
The root's right child is 4, which is less than root 5. Even though 4's children (3, 6) satisfy local constraints, 3 should be > 5 to be in the right subtree. Invalid BST.