input: Write a function to reverse a string.
  assert reverseString('Hello World!')=='!dlroW olleH'
  assert reverseString('Python')=='nohtyP'
  assert reverseString('racecar')=='racecar'
  assert reverseString('')==''
  assert reverseString('12345')=='54321'
output: def reverseString(s):
      return s[::-1]