LeetCode 101: Symmetric Tree

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

Note:
Bonus points if you could solve it both recursively and iteratively.

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

/**
 * 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) {
        // Similar to "Same Tree"
        if (root == null)
            return true;
            
        return isSym(root.left, root.right);
    }
    
    private boolean isSym(TreeNode n1, TreeNode n2)
    {
        if (n1==null || n2==null)
        {
            if (n1==null && n2==null)
                return true;
            else
                return false;
        }
        
        if (n1.val != n2.val)
            return false;
            
        return isSym(n1.left, n2.right) && isSym(n1.right, n2.left);
    }
}