input: Write a function to calculate the sum of all elements in a binary tree. If the tree is empty, return 0.
  
  assert sum_tree(None)==0
  assert sum_tree(TreeNode(1, TreeNode(2, TreeNode(4), TreeNode(5)), TreeNode(3)))==15
  assert sum_tree(TreeNode(10, TreeNode(20, None, TreeNode(30)), TreeNode(40)))==100
  assert sum_tree(TreeNode(7))==7
  assert sum_tree(TreeNode(0, TreeNode(0, TreeNode(0)), 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)