BST Insertion Verifier
Problem Description
A **Binary Search Tree (BST)** is a binary tree where every node satisfies:
- All values in the left subtree are strictly less than the node's value.
- All values in the right subtree are strictly greater than the node's value.
Given the `root` of a binary tree, determine whether it is a valid BST.
**Example 1:**
```
7
/ \
3 9
```
Input: root = [7, 3, 9]
Output: true
**Example 2:**
```
5
/ \
2 8
/ \
4 10
```
Input: root = [5, 2, 8, null, null, 4, 10]
Output: false
Explanation: Node 4 is in the right subtree of 5 but 4 < 5, violating the BST property.
Constraints
- The number of nodes in the tree is in the range [1, 5000].
- Node values are in the range [-10^6, 10^6].
- All node values are unique.
Follow-up
Can you solve this iteratively using an explicit stack to avoid recursion depth issues on a heavily skewed tree?
Hints
Try the problem first. If you get stuck, you can reveal hints one at a time.
Solution
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