Symmetric Tree

Diameter of Binary Tree

Sum of Two Integers

Check whether a binary tree is a mirror image of itself around its center.

Given the `root` of a binary tree, return `true` if it is a mirror of itself (symmetric around its center), or `false` otherwise.

**Input / output**
- Input: `root: TreeNode` (JSON test fixture is a LeetCode-style level-order array, e.g. `[1, 2, 2, 3, 4, 4, 3]`, with `null` for a missing child)
- Output: `boolean`

## Example
```json
Input: {"root":[1,2,2,3,4,4,3]}
Output: true
```

## Constraints
- 0 <= number of nodes <= 1,000
- -100 <= node value <= 100

## Follow-up
Can you solve it both recursively and iteratively with a queue?

The key pattern is: Parallel DFS that mirrors the left/right subtrees against each other. 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(h) for the recursion stack (O(n) worst case on a skewed tree).

All 5 languages below run and submit against the remote judge for this problem — Java/Go/Rust use the same real TreeNode structural-type support the judge added for the rest of the linked-list/tree track.