input: Write a function to determine the sum of all elements in a given binary tree. Return the sum. 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(create_tree([1, 2, 3]))==6
  assert sum_tree(create_tree([5, 4, 8, 11, None, 13, 4]))==45
  assert sum_tree(create_tree([1]))==1
  assert sum_tree(create_tree([-10,9,20,None,None,15,7]))==41
output: def sum_tree(root):
      if root is None:
          return 0
  
      return root.val + sum_tree(root.left) + sum_tree(root.right)