input: 
  from typing import List
  
  def replace_string(input_string: str, target_string: str, replacement_string: str) -> str:
      """ Replace all occurrences of target_string with replacement_string in input_string.
      >>> replace_string("Hello World!", "World", "Python")
      'Hello Python!'
      >>> replace_string("Python is fun, Python is cool!", "Python", "Java")
      'Java is fun, Java is cool!'
      """
  
output:     return input_string.replace(target_string, replacement_string)