input: Write a function to reverse a given string by iterating through each character in reverse order and building a new string from it. If the given string is empty, return the empty string.
  assert reverse_string("Python")=="nohtyP"
  assert reverse_string("Hello")=="olleH"
  assert reverse_string("")==""
  assert reverse_string("12345")=="54321"
output: def reverse_string(s):
      return s[::-1]