input: from typing import List
  
  def replace_string(input_string: str, target_string: str, replacement_string: str) -> str:
      """ Replace all occurrences of a given target string with an alternate string in the input string.
      >>> replace_string("Hello World!", "World", "Python")
      'Hello Python!'
      >>> replace_string("Hello World! Hello World!", "Hello", "Hi")
      'Hi World! Hi World!'
      """
output:     return input_string.replace(target_string, replacement_string)