input: Write a function to count the number of vowels in a given string.
  assert countVowels("hello")==2
  assert countVowels("HELLO")==2
  assert countVowels("world")==1
  assert countVowels("AEIOUaeiou")==10
  assert countVowels("")==0
output: def countVowels(s):
      vowels = "aeiouAEIOU"
      return sum(1 for char in s if char in vowels)