input: Write a function to reverse the characters of a given string. If the input string is empty, the function should return an empty string.
  
  def reverse_string(input_string):
      return input_string[::-1]
  
  assert reverse_string("hello")=="olleh"
  assert reverse_string("Python")=="nohtyP"
  assert reverse_string("")==""
  assert reverse_string("12345")=="54321"
output: def reverse_string(input_string):
      return input_string[::-1]