A message containing letters from A-Z is being encoded to numbers using the following mapping:

‘A‘ -> 1
‘B‘ -> 2
...
‘Z‘ -> 26

Given an encoded message containing digits, determine the total number of ways to decode it.

For example,
Given encoded message "12", it could be decoded as "AB" (1 2) or "L" (12).

The number of ways decoding "12" is 2.

DP，方法感觉有点像Fobinacci

 1 class Solution(object):
 2     def numDecodings(self, s):
 3         if s==‘‘ or s[0]==‘0‘: return 0
 4         r1,r2 = 1,1
 5         for i in range(1,len(s)):
 6             if s[i] == ‘0‘:
 7                 r1 = 0
 8             if s[i-1:i+1]<‘27‘:
 9                 r1,r2 = r1+r2,r1
10             else:
11                 r2= r1
12         return r1
13         

[leetcode DP]91. Decode Ways