input: Write a function to determine the sum of all elements in a binary tree. If the root is None, return 0. Otherwise, return the value of the root plus the sum of all elements in the left subtree plus the sum of all elements in the right subtree.
  
  
  assert sum_tree(None)==0
  assert sum_tree(TreeNode(1, TreeNode(2), TreeNode(3)))==6
  assert sum_tree(TreeNode(1, TreeNode(2, TreeNode(4), TreeNode(5)), TreeNode(3)))==15
  assert sum_tree(TreeNode(10, TreeNode(10), TreeNode(10)))==30
  
output: def sum_tree(root):
      if root is None:
          return 0
      return root.val + sum_tree(root.left) + sum_tree(root.right)