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.

**Example 2:**

1 / \ 2 3

Binary tree `[1,2,3]`

, return false.