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. Example 1:
2 / \ 1 3
Binary tree [2,1,3], return true.
1 / \ 2 3
Binary tree [1,2,3], return false.