Given a binary tree, determine if it is a valid binary search tree (BST).
Assume a 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.
2 / \ 1 3
5 / \ 1 4 / \ 3 6
Explanation: The root node's value is 5 but its right child's value is 4.