91 Decode Ways

91 Decode Ways

dp解法 O(1) space

class Solution:
    # @param {string} s
    # @return {integer}
    def numDecodings(self, s):
        if s == "" or s[0] == "0":
            return 0
        dp0, dp1 = 1, 1
        for i in range(1, len(s)):
            if s[i] == "0":
                if s[i-1] == "0" or s[i-1] >= '3':
                    return 0
                else:
                    dp0, dp1 = dp1, dp0
            else:
                if '10' < s[i-1: i+1] < '27':
                    dp0, dp1 = dp1, dp0 + dp1
                else:
                    dp0 = dp1
        return dp1