input: Write a function to display a greeting specific to a person.
  assert greet('John') == 'Hello John!'
  assert greet('Alice') == 'Hello, stranger!'
  assert greet('John') == 'Hello John!'
  assert greet('Bob') == 'Hello, stranger!'
output: def greet(name):
      if name == 'John':
          return 'Hello John!'
      else:
          return 'Hello, stranger!'