input: Write a function to count the number of vowels in a given string. If the given string is empty or contains no vowels, return 0.
  def count_vowels(string):
      count = 0
      vowels = set("aeiouAEIOU")
      for letter in string:
          if letter in vowels:
              count += 1
      return count
  
  assert count_vowels("") == 0
  assert count_vowels("Hello") == 2
  assert count_vowels("Python") == 1
  assert count_vowels("AEIOU") == 5
  assert count_vowels("bcdfg") == 0
output: def count_vowels(string):
      count = 0
      vowels = set("aeiouAEIOU")
      for letter in string:
          if letter in vowels:
              count += 1
      return count