input: Write a function to replace all occurrences of a given string with an alternate string. The function should return the modified string. If the input string is empty, return an empty string.
  def replace_string(input_string, target_string, replacement_string):
      if input_string == "":
          return ""
      return input_string.replace(target_string, replacement_string)
  
  assert replace_string("Hello World!", "World", "Python") == "Hello Python!"
  assert replace_string("The quick brown fox jumps over the lazy dog", "fox", "cat") == "The quick brown cat jumps over the lazy dog"
  assert replace_string("Sphinx of black quartz, judge my vow", "black", "white") == "Sphinx of white quartz, judge my vow"
  assert replace_string("", "any", "replacement") == ""
  assert replace_string("Repeat Repeat Repeat", "Repeat", "End") == "End End End"
output: def replace_string(input_string, target_string, replacement_string):
      if input_string == "":
          return ""
      return input_string.replace(target_string, replacement_string)