input: Write a function to determine the sum of all elements in a given binary tree. If the tree is empty, return 0.
  assert sum_tree(None)==0
  assert sum_tree(BinaryTreeNode(10, BinaryTreeNode(5), BinaryTreeNode(15)))==30
  assert sum_tree(BinaryTreeNode(7, BinaryTreeNode(3, BinaryTreeNode(1), BinaryTreeNode(4)), BinaryTreeNode(12, BinaryTreeNode(10), BinaryTreeNode(14))))==51
output: def sum_tree(root):
      if root is None:
          return 0
      return root.val + sum_tree(root.left) + sum_tree(root.right)