input: Write a python function to find the maximum depth of a binary tree.
  assert max_depth(root1) == 3  # Assuming root1 corresponds to a tree with depth 3
  assert max_depth(root2) == 4  # Assuming root2 corresponds to a tree with depth 4
  assert max_depth(None) == 0
output: def max_depth(root):
      if root is None:
          return 0
      left_depth = max_depth(root.left)
      right_depth = max_depth(root.right)
      return max(left_depth, right_depth) + 1