Symmetric Tree

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).
For example, this binary tree [1,2,2,3,4,4,3] is symmetric:
1. 1
2. / \
3. 2 2
4. / \ / \
5. 3 4 4 3

But the following [1,2,2,null,3,null,3] is not:
1. 1
2. / \
3. 2 2
4. \ \
5. 3 3

Note:
Bonus points if you could solve it both recursively and iteratively.
Solution:
1. /*
2. * Definition for a binary tree node.
3. * public class TreeNode {
4. * int val;
5. * TreeNode left;
6. * TreeNode right;
7. * TreeNode(int x) { val = x; }
8. * }
9. /
10. public class Solution {
11. public boolean isSymmetric(TreeNode root) {
12. if (root == null) {
13. return true;
14. }
15. return isSymmetric(root.left, root.right);
16. }
17. 18. boolean isSymmetric(TreeNode left, TreeNode right) {
19. if (left == null && right == null) return true;
20. if (left == null || right == null) return false;
21. if (left.val != right.val) return false;
22. return isSymmetric(left.left, right.right) && isSymmetric(left.right, right.left);
23. }
24. }