leetcode——91.解码方法

class Solution:
    def numDecodings(self, s: str) -> int:
        a={}
        if int(s[0])==0:
            return 0
        else:
            a[0]=1
        if len(s)==1:
            return a[0]
        if int(s[0])>2 and int(s[1])==0:
            a[1]=0
            if len(s)>1:
                return 0
        if (int(s[0]+s[1])>26) or (int(s[0])<3 and int(s[1])==0):
            a[1]=1
        elif int(s[0]+s[1])<=26:
            a[1]=2
        #print(a)
        if len(s)==2:
            return a[1]
        for i in range(2,len(s)):
            if s[i-1]==‘0‘ and s[i]==‘0‘:
                return 0
            elif int(s[i-1])>2 and s[i]==‘0‘:
                return 0
            elif (int(s[i-1]+s[i]))>26 or (s[i-1]==‘0‘ and int(s[i])!=0):
                a[i]=a[i-1]
            elif int(s[i-1])<3 and s[i]==‘0‘:
                a[i]=a[i-2]
            elif int(s[i-1]+s[i])<=26:
                a[i]=a[i-1]+a[i-2]
            else:
                a[i]=a[i-1]
        return a[len(s)-1]