can_map.py

def can_map(numbers, letters):
    # print "in can_map for {} and {}".format(numbers, letters)
    if (
        len(numbers) > len(letters) or
        numbers.count('0') > letters.count('A') or
        ('B' in letters and '0' not in numbers)
    ):
        return False

    if len(numbers) == 1:
        mutliple_letters = 'A' in letters and 'B' in letters
        bs_and_zeros = numbers[0] == '0' and 'B' in letters
        return not (mutliple_letters or bs_and_zeros)

    letter = letters[0]
    number = numbers[0]
    if number == '0' and letter == 'B':
        return False
    if can_map(numbers[1:], letters[1:]):
        return True
    if letters[1] != letter:
        # next letter had to belong to next number, previous would be True
        return False
    return can_map(numbers, letters[1:])