Leetcode - Tree - 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 isEqual(root->left, root->right);
    }
    bool isEqual(TreeNode *lhs, TreeNode *rhs)
    {
        if(!rhs&&!lhs)
            return true;
        if((lhs&&!rhs)||(!lhs&&rhs))
            return false;
        return (lhs->val==rhs->val)&&isEqual(lhs->left,rhs->right)&&isEqual(lhs->right,rhs->left);
    }
};