Validate Binary Search Tree
Difficulty: Medium
Category: DSA
Topics: Tree, Depth-First Search, Binary Search Tree, Binary Tree
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.
**Example 1:**
**Input:** root = [2,1,3]
**Output:** true
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**Example 2:**
**Input:** root = [5,1,4,null,null,3,6]
**Output:** false
**Explanation:** The root node's value is 5 but its right child's value is 4.
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**Constraints:**
- The number of nodes in the tree is in the range `[1, 104]`.
- `-231 <= Node.val <= 231 - 1`