Given a binary tree, check whether it is a mirror of itself/symmetric

Below binary tree [1,2,2,3,4,4,3] is symmetric:

    1
   / \
  2   2
 / \ / \
3  4 4  3

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isSymmetric(TreeNode* root) {
         TreeNode* left, *right;
         if (!root)
            return true;
       
         queue<TreeNode*>q1, q2;
         q1.push(root->left);
         q2.push(root->right);
        
        while (!q1.empty() && !q2.empty()) {
            left = q1.front();
            q1.pop();
            right = q2.front();
            q2.pop();
            if (!left && !right)
              continue;
            if (!left || !right)
                return false;
            if (left->val != right->val)
                return false;
            q1.push(left->left);
            q1.push(left->right);
            q2.push(right->right);
            q2.push(right->left); 
        }
        return true;
    }
};


======================================================================================

/**
 * Definition for a binary tree node.
 * 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)
            return true;
         if (!root->left && !root->right)
             return true;
         if (!root->left || !root->right)
            return false;
         queue<TreeNode*>q1, q2;
         q1.push(root->left);
         q2.push(root->right);
       
    }

};