input: Write a function to calculate the sum of all elements in a given binary tree. If the binary 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(BinaryTreeNode(1, BinaryTreeNode(2), BinaryTreeNode(3)))==6
  assert sum_tree(BinaryTreeNode(4, BinaryTreeNode(5, BinaryTreeNode(6)), BinaryTreeNode(7)))==22
  assert sum_tree(BinaryTreeNode(0, None, None))==0
output: def sum_tree(root):
      if root is None:
          return 0
  
      return root.val + sum_tree(root.left) + sum_tree(root.right)