code: def is_valid_sub_bst(node, lower, upper):
      if node is None:
          return True
  
      if (node.val <= lower) or (node.val >= upper):
          return False
  
      # check left
      if not is_valid_sub_bst(node.left, lower, node.val):
          return False
  
      # check right
      if not is_valid_sub_bst(node.right, node.val, upper):
          return False
  
      return True
  
label_desc: Use image node_id 2 for calling a global function with example usage: is_valid_sub_bst(node, lower, upper) and returns: True, True, False, False, False