input: Write a function to reverse the characters of a given string. The function should take a string as an argument and return the string with its characters in reverse order.
  
  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("12345")=="54321"
  assert reverse_string("")==""
output: def reverse_string(input_string):
      return input_string[::-1]