input: Write a function to determine 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
output: def sum_tree(root):
      if root is None:
          return 0
  
      return root.val + sum_tree(root.left) + sum_tree(root.right)