leetcode - 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?

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
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 dfs(root->left,root->right);
    }
private:
	bool dfs(TreeNode *a,TreeNode *b)
	{
		if(a == NULL && b == NULL) return true;
		if((a == NULL && b != NULL) || (a != NULL && b == NULL)) return false;
		return a->val == b->val && dfs(a->left,b->right) && dfs(a->right,b->left);
	}
};