class Node:
  def __init__(self, data):
    self.data = data
    self.left = None
    self.right = None

class BST:
  def __init__(self):
    self.root = None
  
  def _add(self, node, data):
    if node is None:
      return Node(data)
    elif node.data == data:
      return node
    elif data < node.data:
      node.left = self._add(node.left, data)
    else:
      node.right = self._add(node.right, data)
    return node

  def add(self, data):
    self.root = self._add(self.root, data)

  def contains(self, data):
    pass

  def _sum(self, node):
    pass

  def sum(self):
    pass

if __name__ == '__main__':
  bst = BST()
  bst.add(7)
  bst.add(9)
  bst.add(4)
  bst.add(6)
  bst.add(2)
  bst.add(3)
  print(bst.sum())