Decode Ways - Python Leetcode Solution

 A message containing letters from A-Z is being encoded to numbers using the following mapping:

'A' -> 1
'B' -> 2
...
'Z' -> 26

Given a non-empty string containing only digits, determine the total number of ways to decode it.

The answer is guaranteed to fit in a 32-bit integer.

 

Example 1:

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

Example 2:

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

Example 3:

Input: s = "0"
Output: 0
Explanation: There is no character that is mapped to a number starting with '0'. We cannot ignore a zero when we face it while decoding. So, each '0' should be part of "10" --> 'J' or "20" --> 'T'.

Example 4:

Input: s = "1"
Output: 1

class Solution:
    def numDecodings(self, s: str) -> int:
        dp = [0,0]
        prev2 = 1
        prev1 = 0 if s[0] == '0' else 1
        final = 0
        for i in range(2,len(s)+1):
            cur = 0
            if s[i-1] != '0':
                cur += prev1
            two_digit = int(s[i-2:i])
            if two_digit >=10 and two_digit <=26:
                cur += prev2
            prev2,prev1 = prev1,cur
        return prev1
        

TC: O(n) SC: O(1)