class Solution {
public boolean isSymmetric(TreeNode root) {
if (root == null) return true;
return helper(root.left, root.right);
}
private boolean helper(TreeNode left, TreeNode right) {
if (left == null && right == null) return true;
if (left == null || right == null) return false;
return left.val == right.val && helper(left.left, right.right) && helper(left.right, right.left);
}
}
