input: from typing import List
  
  def count_vowels(string: str) -> int:
      """ Count the number of vowels in a given string.
      >>> count_vowels("Hello World")
      3
      >>> count_vowels("Why so serious?")
      4
      """
output:     vowels = "aeiouAEIOU"
      count = 0
      for char in string:
          if char in vowels:
              count += 1
      return count