input: Write a function to determine the sum of all elements in a given binary tree.
  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(Tree(1, Tree(2, Tree(4), Tree(5)), Tree(3)))==15
  assert sum_tree(Tree(1, None, Tree(2)))==3
  assert sum_tree(Tree(5, Tree(3), Tree(7)))==15
output: def sum_tree(root):
      if root is None:
          return 0
  
      return root.val + sum_tree(root.left) + sum_tree(root.right)