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.

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isSymmetric(TreeNode *root) 
    {
        if(root==NULL)
            return true;
        return isMirror(root->left,root->right);
    }
    bool isMirror(TreeNode* left,TreeNode* right)
    {
        if(left==NULL && right==NULL)
            return true;
        if(left==NULL || right==NULL)
            return false;
        if(left->val!=right->val)
            return false;
        return isMirror(left->left,right->right) && isMirror(left->right,right->left);
    }
};