Given the `root` of a binary tree, check whether it is a mirror of itself (i.e., symmetric around its center).
**Example 1:**
```
Input: root = [1,2,2,3,4,4,3]
Output: true
```
**Example 2:**
```
Input: root = [1,2,2,null,3,null,3]
Output: false
```
Could you solve it both recursively and iteratively?