input: Write a function to reverse a given string. The function should take a string as input and return the string with its characters in reverse order. If the input string is empty, the function should return an empty 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(s):
      return s[::-1]