Symmetric Tree (LeetCode Tree)

Question: Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).
For example, this binary tree is symmetric:
    1
   / \
  2   2
 / \ / \
3  4 4  3
But the following is not:
    1
   / \
  2   2
   \   \
   3    3

Idea: Depth-first search. I used preorder traversal, compare current, then traverse (left.left, right.right) and (left.right, right.left) at the same time. Once a mismatch happens, stops and return.

Time: O(n) Space: O(lgn)

Code:

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