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.
Examples
Example 1
Input:
root = [2,1,3]Output:
trueExample 2
Input:
root = [5,1,4,null,null,3,6]Output:
falseThe 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, 10^4]. - •
-2^31 <= Node.val <= 2^31 - 1
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