input: Write a python function to convert temperatures between Celsius and Fahrenheit.
  assert celsius_to_fahrenheit(0) == 32
  assert celsius_to_fahrenheit(100) == 212
  assert fahrenheit_to_celsius(32) == 0
  assert fahrenheit_to_celsius(212) == 100
output: def celsius_to_fahrenheit(celsius):
      return (celsius * 9/5) + 32
  
  def fahrenheit_to_celsius(fahrenheit):
      return (fahrenheit - 32) * 5/9