打卡群刷题总结0801——解码方法

class Solution(object):
    def numDecodings(self, s):
        """
        :type s: str
        :rtype: int
        """
        if len(s) == 0:
            return 0
        if s[0] == '0':
            return 0

        # 00, 30, 40...90: return 0
        # 10, 20: dp[i] = dp[i - 2]
        # 27-99(exclude 30, 40, ...)): dp[i] = dp[i - 1]
        # 1-26(exclude 10, 20): dp[i] = dp[i - 1] + dp[i - 2]
        dp = [0] * (len(s) + 1)
        dp[0] = 1
        dp[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:
                    dp[i + 1] = dp[i - 1]
            elif s[i - 1] == '0' or s[i - 1] >= '3':
                dp[i + 1] = dp[i]
            elif s[i - 1] == '2' and s[i] >= '7':
                dp[i + 1] = dp[i]
            else:
                dp[i + 1] = dp[i] +dp[i - 1]
        print(dp)
        return dp[-1]