leetcode 91 解码方法

class (object):
    def numDecodings(self, s):
        """
        :type s: str
        :rtype: int
        """
        cnt = 0
        if not s:
            return 0
        dp = [0] * (len(s)+1)
        dp[-1] = 1
        for i in range(len(s)-1, -1, -1):
            if s[i] == '0':
                continue
            dp[i] = dp[i+1]
            if s[i] != '0' and i+2 <= len(s) and 10<=int(s[i:i+2])<= 26:
                dp[i] += dp[i+2]
        return dp[0]