input: Write a function to reverse the characters of a given 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'
output: def reverse_string(input_string):
      return input_string[::-1]