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("The quick brown fox jumps over the lazy dog.", "fox", "cat")
      "The quick brown cat jumps over the lazy dog."
      """
output:     return input_string.replace(target_string, replacement_string)