input: Write a function to reverse a given string using an optimized approach. The function should take a string as input and return the reversed string. If the input string is empty, the function should return an empty string.
  assert reverse_string("hello")=="olleh"
  assert reverse_string("world")=="dlrow"
  assert reverse_string("python")=="nohtyp"
  assert reverse_string("")==""
output: def reverse_string(s):
      return s[::-1]