Here is my AC code, I really eager to know the time complexity of this solution..

I think it's O(n), since we do the (n/2) comparison at most. But I am not sure...

```
public boolean isSymmetric(TreeNode root) {
if(root == null) return true;
return dfs(root.left, root.right);
}
private boolean dfs(TreeNode left, TreeNode right) {
if(left == null && right == null) return true;
if(left == null || right == null) return false;
if(left.right)
/* Follow the symmetric defination */
return (left.val == right.val) && dfs(left.right, right.left) && dfs(left.left, right.right);
}
```