input: Write a function to reverse a string by iterating through its characters from the last character to the first and concatenating them into a new string. Return the new string which represents the reversed 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('a')=='a'
output: 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