input: Write a function to determine the sum of all elements in a binary tree. If the given tree is empty then return 0.
  assert sum_tree(None) == 0
  assert sum_tree(create_tree([1,2,3])) == 6
  assert sum_tree(create_tree([5,10,None,1,2])) == 18
  assert sum_tree(create_tree([7])) == 7
  assert sum_tree(create_tree([1,2,None,4,5])) == 12
output: def sum_tree(root):
      if root is None:
          return 0
      return root.val + sum_tree(root.left) + sum_tree(root.right)