input: Write a function to replace all occurrences of a given string with an alternate 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("Hello World!", "o", "a")=="Hella Warald!"
  assert replace_string("Hello World!", "Hello", "Hi")=="Hi World!"
  assert replace_string("", "World", "Python")==""
  assert replace_string("Hello World!", "Pizza", "Burger")=="Hello World!"
output: def replace_string(input_string, target_string, replacement_string):
      if input_string == "":
          return ""
      return input_string.replace(target_string, replacement_string)