input: def replace_string(input_string: str, target_string: str, replacement_string: str) -> str:
      """ Replace all occurrences of the target string with the replacement string in the input string.
      >>> replace_string("Hello World!", "World", "Python")
      "Hello Python!"
      >>> replace_string("This is a test", "is", "was")
      "Thwas was a test"
      """
output:     return input_string.replace(target_string, replacement_string)