input: Write a function to calculate the sum of all elements in a given binary tree. If the given tree is empty then 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(Node(1, Node(2, Node(4), Node(5)), Node(3)))==15
  assert sum_tree(Node(10, None, Node(20)))==30
  assert sum_tree(Node(0))==0
  assert sum_tree(Node(-1, Node(-2), Node(-3)))==-6
output: def sum_tree(root):
      if root is None:
          return 0
  
      return root.val + sum_tree(root.left) + sum_tree(root.right)