input: Write a function to reverse a given string. If the string is empty, return an empty string.
  def reverse_string(input_string):
      reversed_string = ""
      for i in range(len(input_string)-1, -1, -1):
          reversed_string += input_string[i]
      return reversed_string
  assert reverse_string("Python")=="nohtyP"
  assert reverse_string("HelloWorld")=="dlroWolleH"
  assert reverse_string("")==""
output: def reverse_string(input_string):
      return input_string[::-1]