input: Write a python function to replace all occurrences of a substring in a string with a new substring using recursion.
  assert replace("Hello World", "World", "John") == "Hello John"
  assert replace("banana", "a", "o") == "bonono"
  assert replace("abababab", "ab", "ba") == "bababa"
output: def replace(s, old, new):
      if old == "":
          return s
      index = s.find(old)
      if index == -1:
          return s
      return s[:index] + new + replace(s[index + len(old):], old, new)