Solve the Medium-level Validate Binary Search Tree coding interview problem online in Python & JavaScript. Run your code against real test cases in the browser and study worked examples. It covers binary tree, binary search tree, and depth first search. Read the step-by-step solution and complexity analysis. Java starter code is also included for reference (no in-browser JVM yet, so it can't be run here).

Validate Binary Search Tree – Solve Online in Python & JavaScript

Binary Tree Level Order Traversal

Same Tree

Check whether a binary tree satisfies strict BST ordering.

Return true if every node in `root` satisfies the strict binary-search-tree ordering rule.

## Example
```json
Input: {"root":[2,1,3]}
Output: true
```

## Constraints
- 1 <= number of nodes <= 10000
- -2^31 <= node value <= 2^31 - 1

## Follow-up
Why is checking only each node's immediate children insufficient?

## Execution note
Java/Go/Rust don't support this problem's ListNode/TreeNode structure yet — the starter is reference-only. Try Python or JavaScript to run and submit.

The key pattern is: DFS carries the full valid value range. Identify the state and invariant before coding.

Recognize the Pattern

Advance one state transition at a time. Mark or update state before exploring dependent work.

Build the Algorithm

Test empty or minimal input, skewed shapes, duplicates where allowed, and impossible outcomes.