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
Explanation: The tree is symmetric. The left subtree [2, 3, 4] is a mirror of the right subtree [2, 4, 3].
Example 2:
Input: root = [1, 2, 2, null, 3, null, 3]
Output: false
Explanation: The tree is not symmetric. Both sides have a 3 on the right, but a mirror would require a 3 on the left of the right subtree.
Example 3:
Input: root = [1]
Output: true
[1, 1000]-100 <= Node.val <= 100