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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 3Binary tree

`[2,1,3]`

, return true.
**Example 2:**

1 / \ 2 3Binary tree

`[1,2,3]`

, return false.
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