007.py

"""
Problem:

Given the mapping a = 1, b = 2, ... z = 26, and an encoded message, count the number of
ways it can be decoded.

For example, the message '111' would give 3, since it could be decoded as 'aaa', 'ka',
and 'ak'.

You can assume that the messages are decodable. For example, '001' is not allowed.
"""


def count_decoding(digits: str) -> int:
    len_digits = len(digits)
    # dynamic Programming table
    count = [0 for _ in range(len_digits + 1)]
    # base cases
    count[0] = 1
    count[1] = 1

    for i in range(2, len_digits + 1):
        count[i] = 0
        # if the last digit is not 0, then last digit must add to the number of words
        if digits[i - 1] > "0":
            count[i] = count[i - 1]
        # if the number formed by the last 2 digits is less than 26, its a valid
        # character
        if digits[i - 2] == "1" or (digits[i - 2] == "2" and digits[i - 1] < "7"):
            count[i] += count[i - 2]
    return count[len_digits]


if __name__ == "__main__":
    print(count_decoding("81"))
    print(count_decoding("11"))
    print(count_decoding("111"))
    print(count_decoding("1311"))
    print(count_decoding("1111"))


"""
SPECS:

TIME COMPLEXITY: O(n)
SPACE COMPLEXITY: O(n)
"""