input: Write a function to search a given key in a binary search tree.
  assert search(r, 70).val == 70
  assert search(r, 25) == None
  assert search(r, 60).val == 60
output: def search(root, key):
      if root is None or root.val == key:
          return root
      if key < root.val:
          return search(root.left, key)
      return search(root.right, key)