LeetCode - Symmetric Tree

/**
* Definition for binary tree
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
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) {
return true;
}
if((left!=null&&right==null) || (left==null&&right!=null) ||
left.val!=right.val) {
return false;
}
return dfs(left.left, right.right) && dfs(left.right, right.left);
}
}