Symmetric Tree

/**
* Definition for a binary tree node.
* 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 isSymmetricHelper(root.left, root.right);
}

public boolean isSymmetricHelper(TreeNode left, TreeNode right) {
if (left == null && right == null) {
return true;
} else if (left != null && right != null) {
if (left.val != right.val) {
return false;
}
return isSymmetricHelper(left.left, right.right) && isSymmetricHelper(left.right, right.left);
} else {
return false;
}
}
}