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