Given a binary tree, determine if it is a valid binary search tree (BST).
Assume a BST is defined as follows:
<li>The left subtree of a node contains only nodes with keys <strong>less than</strong> the node's key.</li>
<li>The right subtree of a node contains only nodes with keys <strong>greater than</strong> the node's key.</li>
<li>Both the left and right subtrees must also be binary search trees.</li>
Example 1:
2 / \ 1 3 Input: [2,1,3] Output: true
Example 2:
5 / \ 1 4 / \ 3 6 Input: [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.