input: Write a function to reverse a given string. If the string is empty, return an empty string. The function should return the reversed string.
  
  assert reverse_string("Python")=="nohtyP"
  assert reverse_string("Hello")=="olleH"
  assert reverse_string("12345")=="54321"
  assert reverse_string("")==""
output: def reverse_string(s):
      return s[::-1]