input: Write a function to calculate the sum of all elements in a binary tree. If the given root node is None then return 0.
  assert sum_tree(None)==0
  assert sum_tree(TreeNode(5, TreeNode(3, TreeNode(2), TreeNode(4)), TreeNode(8, TreeNode(7), TreeNode(9))))==38
  assert sum_tree(TreeNode(10, None, TreeNode(15, TreeNode(12), TreeNode(20))))==57
  assert sum_tree(TreeNode(1, TreeNode(2, TreeNode(3), TreeNode(4)), TreeNode(5, TreeNode(6), TreeNode(7))))==28
  assert sum_tree(TreeNode(0))==0
output: def sum_tree(root):
      if root is None:
          return 0
      return root.val + sum_tree(root.left) + sum_tree(root.right)