# Problem Definition

## Description

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 sub-trees must also be binary search trees.

Example :

```plaintext
Input:
   1
  /  \
 2    3

Output : 0 or False


Input:
  2
 / \
1   3

Output : 1 or True
```

Return 0 / 1 ( 0 for false, 1 for true ) for this problem

## Notes

1. [YT - CodeSchool - Check if a binary tree is binary search tree or not](https://www.youtube.com/watch?v=yEwSGhSsT0U)
1. [YT - Tushar Roy - Check if Binary Tree is Binary Search Tree](https://www.youtube.com/watch?v=MILxfAbIhrE)