Given the root of a binary tree represented as a level-order array (with `null` for missing nodes), check whether it is a mirror of itself (i.e., symmetric around its center).
For example, `[1, 2, 2, 3, 4, 4, 3]` represents:
```
1
/ \
2 2
/ \ / \
3 4 4 3
```
This tree is symmetric.
But `[1, 2, 2, null, 3, null, 3]` is not:
```
1
/ \
2 2
\ \
3 3
```
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
Constraints
- The number of nodes in the tree is in the range [1, 1000].
- -100 <= Node.val <= 100