input: Write a function to determine the sum of all elements in a given binary tree. If the tree is empty (i.e., the root is None), 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(5, TreeNode(10), None))==15
  assert sum_tree(TreeNode(7))==7
  assert sum_tree(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)