Solve the Medium-level Binary Tree Level Order Traversal 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, breadth first search, and queue. 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).

Binary Tree Level Order Traversal – Solve Online in Python & JavaScript

Lowest Common Ancestor of a BST

Validate Binary Search Tree

Return binary-tree values grouped by breadth-first level.

Return the values of binary tree `root` level by level from left to right.

## Example
```json
Input: {"root":[3,9,20,null,null,15,7]}
Output: [[3],[9,20],[15,7]]
```

## Constraints
- 0 <= number of nodes <= 2000
- -1000 <= node value <= 1000

## Follow-up
How can queue length mark the boundary between levels?

## 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: BFS processes one queue level at a time. 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.