https://leetcode.com/problems/symmetric-tree/
Given the root of a binary tree, check whether it is a mirror of itself (i.e., symmetric around its center).
example1:
Input: root = [1,2,2,3,4,4,3]
Output: true
example2:
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
Follow up: Could you solve it both recursively and iteratively?