91. Decode Ways

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).

Code：

class Solution(object):
    def numDecodings(self, s):
        """
        :type s: str
        :rtype: int
        """
        if not s:
            return 0

        dp = [0 for x in range(len(s) + 1)]
        dp[0] = 1
        dp[1] = 1 if 0 < int(s[0]) <= 9 else 0

        for i in range(2, len(s) + 1):
            if 0 < int(s[i-1:i]) <= 9:
                dp[i] += dp[i - 1]
            if s[i-2:i][0] != '0' and int(s[i-2:i]) <= 26:
                dp[i] += dp[i - 2]
        return dp[len(s)]