Medium: Binary Tree Level Order Traversal — solve it online. Python & JavaScript run locally; Java, Go & Rust need sign-in.

Binary Tree Level Order Traversal – Solve Online

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?

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.

Check Edge Cases

Time: O(n). Space: O(n).

Java/Go/Rust below are reference solutions for learning the pattern in those languages; only Python and JavaScript can be run and submitted for this problem, since the remote judge doesn't support the TreeNode structure yet.