input: Write a function to replace all occurrences of a given string with an alternate string.
  
  def replace_string(input_string, target_string, replacement_string):
      return input_string.replace(target_string, replacement_string)
  
  assert replace_string("Hello World!", "World", "Python") == "Hello Python!"
  assert replace_string("Python is fun, Python is great!", "Python", "Java") == "Java is fun, Java is great!"
  assert replace_string("No replacement here", "nothing", "something") == "No replacement here"
  assert replace_string("", "World", "Python") == ""
  assert replace_string("HelloHelloHello", "Hello", "Hi") == "HiHiHi"
output: def replace_string(input_string, target_string, replacement_string):
      return input_string.replace(target_string, replacement_string)