Validate Binary Search Tree (medium)

## Problem Statement

Determine if a given binary tree is a binary search tree (BST). In a BST, for each node:

- All nodes to its left have values less than the node's value.
- All nodes to its right have values greater than the node's value.

### Example Generation

**Example 1:**

**Input:**[5,3,7]**Expected Output:**true**Justification:**The left child of the root (3) is less than the root, and the right child of the root (7) is greater than the root. Hence, it's a BST.

**Example 2:**

**Input:**[5,7,3]**Expected Output:**false**Justification:**The left child of the root (7) is greater than the root, making it invalid.

**Example 3:**

**Input:**[10,5,15,null,null,12,20]**Expected Output:**true**Justification:**Each subtree of the binary tree is a valid binary search tree. So, a whole binary tree is a valid binary search tree.

## Try it yourself

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Python3

Python3

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