bool checkNodes(struct TreeNode* a, struct TreeNode* b)
{
    if(a == NULL && b == NULL)
    {
        return true;
    }

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