code: def mergeTrees(self, t1, t2):
      """
      :type t1: TreeNode
      :type t2: TreeNode
      :rtype: TreeNode
      """
      if t1 is None:
          return t2
      if t2 is None:
          return t1
      t1.val += t2.val
      t1.left = self.mergeTrees(t1.left, t2.left)
      t1.right = self.mergeTrees(t1.right, t2.right)
      return t1
  
label_desc: Use image node_id 1 for calling the Solution obj's underlying member method code with example usage: obj.mergeTrees(t1, t2) and returns: t1, t2, t1