input: Write a function to calculate the sum of all elements in a binary tree. If the tree is empty, return 0.
  def sum_tree(root):
      if root is None:
          return 0
      return root.val + sum_tree(root.left) + sum_tree(root.right)
  
  assert sum_tree(None)==0
  assert sum_tree(Node(1, None, None))==1
  assert sum_tree(Node(1, Node(2, Node(4, None, None), Node(5, None, None)), Node(3, Node(6, None, None), Node(7, None, None))))==28
output: def sum_tree(root):
      if root is None:
          return 0
      return root.val + sum_tree(root.left) + sum_tree(root.right)