题解 | 解密

#
# 
# @param s string字符串 
# @return int整型
#
class Solution:
    def numDecodings(self , s ):
        a=1
        b=1
        dp=1
        if s == '' or s[0] == '0':  
            return 0
        else:
            for i in range(1,len(s)):
                if int(s[i]) == 0 and(int(s[i-1] )> 2 or int(s[i-1]) ==0):
                    return 0  
                elif int(s[i]) != 0 and (int(s[i-1]) == 1 or (int(s[i-1])==2 and int(s[i]) <7) ) :
                    dp = a+b
                else:
                    dp= b
                a , b = b , dp
            return dp