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