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 visit(TreeNode *left, TreeNode *right)
    {
        if (left == NULL && right == NULL)
        {
            return true;
        }

        if (left == NULL || right == NULL || left->val != right->val)
        {
            return false;
        }
    
        return visit(left->left, right->right) && visit(left->right, right->left);
    }
    
    bool isSymmetric(TreeNode *root) {
        if (root == NULL)
        {
            return true;
        }

        return visit(root->left, root->right);
    }
};