input: Write a python function to find the nth largest element in a binary search tree.
  assert nth_largest(root1, 2) == expected_value1
  assert nth_largest(root2, 3) == expected_value2
  assert nth_largest(root3, 1) == expected_value3
output: def nth_largest(root, n):
      def reverse_inorder_traversal(node):
          if not node:
              return []
          return reverse_inorder_traversal(node.right) + [node.val] + reverse_inorder_traversal(node.left)
      
      sorted_elements = reverse_inorder_traversal(root)
      if n <= len(sorted_elements):
          return sorted_elements[n-1]
      return None