input: Write a python function to extract numbers from a string.
  assert extract_numbers("This is 3 string with 4 numbers") == [3, 4]
  assert extract_numbers("abc123xyz456") == [1, 2, 3, 4, 5, 6]
  assert extract_numbers("no digits here!") == []
  assert extract_numbers("1001 nights") == [1, 0, 0, 1]
output: def extract_numbers(s):
      return [int(char) for char in s if char.isdigit()]