if(root1 == NULL)
      return root2;
    if(root2 ==NULL)
        return root1;
    
    root1->val = root1->val +root2->val;
    
    root1->left = mergeTrees(root1->left ,root2->left );
    root1->right = mergeTrees(root1->right, root2->right );
    
    return root1;
}