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.

Example 1:

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

Example 2:

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

class Solution:
    def numDecodings(self, s: str) -> int:        
        if s[0] == '0': return 0
        
#         N = len(s)
#         dp = [1 for i in range(N+1)]
        
        for i in range(1, len(s)):
            if s[i] == '0':
                # dp[i] = 0
                b = 0
            if int(s[i-1:i+1]) <= 26:
                # dp[i+1] = dp[i] + dp[i-1]
                c = b+a
            else:
                # dp[i+1] = dp[i]
                c = b
            a=b
            b=c
            
        return dp[-1]