Decode Ways Python Solution with Tests

lcpy gen -n 91   # by problem number
lcpy gen -s decode_ways   # by problem name

Input: s = "12"
Output: 2
Explanation: "12" could be decoded as "AB" (1 2) or "L" (12).

Input: s = "226"
Output: 3
Explanation: "226" could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).

Input: s = "06"
Output: 0
Explanation: leading zero makes it invalid.

class Solution:
    # Time: O(n)
    # Space: O(1)
    def num_decodings(self, s: str) -> int:
        if not s:
            return 0

        num_ways_two_steps_behind: int = 1
        num_ways_one_step_behind: int = 0 if s[0] == "0" else 1

        for index in range(1, len(s)):
            current_char: str = s[index]
            previous_char: str = s[index - 1]

            current_num_ways: int = 0

            if current_char != "0":
                current_num_ways += num_ways_one_step_behind

            two_digit_value: int = int(previous_char + current_char)
            if previous_char != "0" and 10 <= two_digit_value <= 26:
                current_num_ways += num_ways_two_steps_behind

            num_ways_two_steps_behind, num_ways_one_step_behind = (
                num_ways_one_step_behind,
                current_num_ways,
            )

        return num_ways_one_step_behind