Given the root of a binary tree, return True if the tree is a mirror of itself around a
vertical line through the root: the left subtree must be the mirror image of the right subtree.
Examples
1
/ \
2 2
/ \ / \
3 4 4 3
Input: root = build_tree([1, 2, 2, 3, 4, 4, 3])
Output: True
1
/ \
2 2
\ \
3 3
Input: root = build_tree([1, 2, 2, None, 3, None, 3])
Output: False
Constraints
0 <= number of nodes <= 2000-10**4 <= node.val <= 10**4- Target complexity: O(n) time.
Goals
- Compare two subtrees as mirror images
- Pair left-with-right and right-with-left in the recursion
- Handle an empty tree and a single node