mediumTrees
Validate Binary Search Tree
## Problem
Given the root of a binary tree, *determine if it is a valid binary search tree (BST)*.
A **valid 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.
> **Input format:** Level-order list where `None` represents a missing node.
Given the root of a binary tree, *determine if it is a valid binary search tree (BST)*.
A **valid 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.
> **Input format:** Level-order list where `None` represents a missing node.
Examples
Input
root = [2,1,3]
Output
true
1 < 2 < 3 — valid BST.
Input
root = [5,1,4,null,null,3,6]
Output
false
Root 5 has right child 4 which violates BST.
Constraints
The number of nodes in the tree is in the range [1, 10^4].
-2^31 <= Node.val <= 2^31 - 1
Python
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