decode ways

class (object):
    def numDecodings(self, s):
        """
        :type s: str
        :rtype: int
        """
        if not s:
            return 0
        if s[0] == "0":
            return 0
        if len(s) == 1:
            return 1
        dp = [0 for _ in range(len(s))] 
        
        # obviously, dp[0] = 1
        dp[0] = 1
        if "1" <= s[:2] <= "26" and s[1] != "0":
            dp[1] = 2
        elif s[1] == "0" and s[0] not in "12":
            return 0
        else:
            dp[1] = 1
        
        for i in range(2, len(s)):
            if s[i] == "0" and s[i-1] not in "12":
                return 0
            elif s[i] == "0":
                dp[i] = dp[i-2]
            elif "11" <= s[i-1:i+1] <= "26":
                dp[i] = dp[i-1] + dp[i-2]
            else:
                dp[i] = dp[i-1]
        
        return dp[-1]