input: Write a function to reverse a given string. The function should take a string argument and return the characters of the string in reverse order. If the given string is empty then 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("hello")=="olleh"
  assert reverse_string("Python")=="nohtyP"
  assert reverse_string("")==""
  assert reverse_string("12345")=="54321"
  assert reverse_string("racecar")=="racecar"
output: def reverse_string(input_string):
      return input_string[::-1]